External Bootable SSD Late 2015 iMac

[plasticbag01]

Hi All,

I'm looking to buy an external SSD and make it bootable for my late 2015 iMac. I'm open to any suggestions, but I'm thinking of this as a single dedicated drive (I don't need multidrive units). It's a 5k display, with USB 3 and Thunderbolt 2 ports. I'm looking for the fastest option available as it's going to a boot drive.

Thunderbolt 2 wasn't wildly popular and I'm having a hard time finding any drives, am I wasting my time? I don't mind putting drives into other enclosures, it doesn't have to be one stop shopping.

If anyone has had any success (or failures) with the above request please let me know. Any additional advice as well would be great.

Thanks in advance!

 

[jmiddel]

USB-3 will be fine, so just get a SATA to USB-3 adapter and an appropriately sized SSD, clone the internal to the external using CarbonCopyCloner and you’re good to go.

 

[ChrisA]

Buy a USB 3 SSD. Format it as APFS, install MacOS then use Apple's migration assistant to move you data and apps. Then change the default boot drive and reboot.

After using your new system long enough that you are sure there are now problems. and ADTER the first full Time Machine backup, then, erase the Mac Internal drive, make it also APFS and use it for storing bulk data that dose not need to be on the SSD.

The above is exactly what I did with my iMac. Also if you have no already done so install as much RAM as will fit the the iMac, just max it out.

YouTime machine backup drive does not need to be USB 3, it can be slow but you may as well use USB 3. Make sure this TM drive is at least 2X larger then al to data you have or will have

I would not erase (aka "reformat') the internal drive until after you are running a few data on the external SSD and Time Machine has finished it's first complete backup of the new SSD. Maybe even wait until our cloud backup is done? (If you don't have this, sign up for it. Backblaze is only $5 per month.)

One issue is keeping a usable backup while you make the switch. You have your old data i-on the old drive and I presume a Time Machine backup of this. To avoid accidents, unplug the tm drive and lock it in a drawer until after the transition is complete. This will prevent human error which is easier than you r think to do. Put the old TM drive is your desk at work, or see place far away. Buy a new TM drive for your new setup.

 

[plasticbag01]

Thanks so much for the replies, I'm surprised Thunderbolt is not recommended. I thought it was a faster option than USB 3, which would be important given it's a boot drive. Glad to hear USB 3 works.

 

[Moonjumper]

Thanks so much for the replies, I'm surprised Thunderbolt is not recommended. I thought it was a faster option than USB 3, which would be important given it's a boot drive. Glad to hear USB 3 works.

While Thunderbolt is potentially better, price and availability gives USB3 significant advantages and only a small performance downside.

 

[weckart]

I bought a cheap TB1 used drive and replaced the internal spinner for an SSD to use as the boot drive for my sister's 2015 iMac. The internal drive was a very slow 5400rpm. It wasn't that expensive to do. The main advantage is that TRIM is supported over TB but not over USB.

 

[the-bigwig]

Was in the same boat as you. Went with USB 3.0 enclosure with UASP support. What an improvement! About 370mbps/370mbps speeds versus 100mbps before. I suppose that's about 25% slower than Thunderbolt but well, you just can't notice it at all. The differences are imo not worth the cost of TB enclosure.

 

[iMacGuru]

I, too, am using an external SSD plus an external HDD for the Time Machine.All run great using USB 3. My internal fusion drive sits there awaiting some use. I'm running High Sierra because I was having continuous bouncing beach balls using Mojave.

 

[SaSaSushi]

Probably too late for the OP but $85 for the Delock 42510 Thunderbolt enclosure plus the price of a Thunderbolt cable was well worth it for me when invested in one about 5 years ago to boot my Late 2013 iMac (at the time).

I've since replaced that iMac with a 2017 iMac with 512GB SSD have been continuing to use the Delock to run BootCamp Windows. I had to invest in an Apple Thunderbolt 2 to Thunderbolt 3 cable but it was worth it. It's a great enclosure.

I definitely recommend Thunderbolt for an external boot SSD. Thunderbolt is faster and allows the use of TRIM. Even for that alone I recommend it.

 

[multitudes]

I agree with SaSaSushi on this one. The 2015 MBP has a very fast PCIe M.2 SSD which clocks at more than 1,400MBps for writes and more than 1,300MBps for reads. USB C has a max nominal bandwidth of 10Gbps which will be less than 1250 MB/s. This will be still fast but compared to the speed of the internal drive is a bottleneck. Now consider that Thunderbolt 3 has a 40 Mbps equal to max bandwidth of 5000 MB/s.
I read a nice article on Anandtech about this. They do not recommend the Samsung X5 due to thermal throttling issues but the TEKQ Rapide TB3 seems quite amazing.

 

[ignatius345]

Hi All,

I'm looking to buy an external SSD and make it bootable for my late 2015 iMac. I'm open to any suggestions, but I'm thinking of this as a single dedicated drive (I don't need multidrive units). It's a 5k display, with USB 3 and Thunderbolt 2 ports. I'm looking for the fastest option available as it's going to a boot drive.

Thunderbolt 2 wasn't wildly popular and I'm having a hard time finding any drives, am I wasting my time? I don't mind putting drives into other enclosures, it doesn't have to be one stop shopping.

If anyone has had any success (or failures) with the above request please let me know. Any additional advice as well would be great.

Thanks in advance!

Hey, how did this go? I've got very similar hardware to yours and have been considering the same upgrade. Thanks!

 

[Bremse]

Here is what I just did: 512 GB NVMe-based external Thunderbolt 3 SSD (was just $100 at Amazon). I connected it to a HP portable Thunderbolt 3 dock which is connected to an Apple TB3 to TB2 Adapter which is plugged into the iMac with an Apple TB2 cable. It’s a base model 27” late 2015 iMac with standard hard drive. I get read and write speeds of right around 1250 MB/sec. Booting works and no sleep issues either. It’s much faster than a USB3 drive and a ton faster than the internal spinner. I already had one but the HP docks are on eBay for cheap (get the travel version without attached TB3 cable - I’ve seen them as low as $50 with power adapter) and I got the TB adapter and cable off FB marketplace for a combined $25. Not the most elegant solution but it works very well and wasn’t too expensive. At some point I will upgrade the ssd in the enclosure with a larger one (it uses standard NVME blades). Note that NVMe is not supported in older MacOS versions but I’m running Catalina without issues. I would go with a thunderbolt drive of some sort due to the increased throughout. Another option I considered but would have been more expensive is a PCIe enclosure (either TB2 or TB3 plus the Apple adapter). You can install a PCIe to NVMe Adapter card and a standard NVMe blade drive. Speed should be about the same as it’s limited by TB2 to about 1200-1300 MB/s.
The HP dock also provides extra USB ports as well as Displayport but I haven’t tried a display yet (USB ports work).

Edit: You need a dock to power the TB3 drive since the drive is bus powered but the Apple TB2/3 adapter doesn’t provide power to the TB3 device. Also, the Apple adapter appears to be the only one you can use “backwards” to connect TB3 devices to TB1/2 Macs. Other adapters only allow connection of TB1/2 devices to TB3 Macs.

 

[1ampaulPH]

Here is what I just did: 512 GB NVMe-based external Thunderbolt 3 SSD (was just $100 at Amazon). I connected it to a HP portable Thunderbolt 3 dock which is connected to an Apple TB3 to TB2 Adapter which is plugged into the iMac with an Apple TB2 cable. It’s a base model 27” late 2015 iMac with standard hard drive. I get read and write speeds of right around 1250 MB/sec. Booting works and no sleep issues either. It’s much faster than a USB3 drive and a ton faster than the internal spinner. I already had one but the HP docks are on eBay for cheap (get the travel version without attached TB3 cable - I’ve seen them as low as $50 with power adapter) and I got the TB adapter and cable off FB marketplace for a combined $25. Not the most elegant solution but it works very well and wasn’t too expensive. At some point I will upgrade the ssd in the enclosure with a larger one (it uses standard NVME blades). Note that NVMe is not supported in older MacOS versions but I’m running Catalina without issues. I would go with a thunderbolt drive of some sort due to the increased throughout. Another option I considered but would have been more expensive is a PCIe enclosure (either TB2 or TB3 plus the Apple adapter). You can install a PCIe to NVMe Adapter card and a standard NVMe blade drive. Speed should be about the same as it’s limited by TB2 to about 1200-1300 MB/s.
The HP dock also provides extra USB ports as well as Displayport but I haven’t tried a display yet (USB ports work).

Edit: You need a dock to power the TB3 drive since the drive is bus powered but the Apple TB2/3 adapter doesn’t provide power to the TB3 device. Also, the Apple adapter appears to be the only one you can use “backwards” to connect TB3 devices to TB1/2 Macs. Other adapters only allow connection of TB1/2 devices to TB3 Macs.

Been looking for this! This is what I am planning to do. You mind posting the exact model of your HP dock and which NVMe enclosure you used? Does USB Type C work on the dock through your HP T3 dock?

 

[Bremse]

The dock is HP model HSTNN-CX01. This model is a flat slim brick with several USB ports and two Displayports (haven’t tried DP but USB works). You can find them on eBay for about $25 without the power adapter or about $50 with (power adapters alone are around $15-$50 as well so need to shop around a bit). Mine has a 150W power adapter but they have a 65W model as well that should be fine and might be cheaper.

HP also makes a bigger cubes shaped dock which seems to sell for a bit more. I haven’t tried that one but think it will work as well. That model has what appears to be an attached TB3 cable but the instructions say it can be removed although I’m not sure how much room there is for other TB3 plugs (the connector is under a cover). This would be fine if your TB3 Drive has a detachable cable but mine doesn’t. The Apple TB adapter has a TB3 connector so you need to have a female socket to connect it.

I haven’t tried USB-C (or 3.2). My dock has two C style sockets but I’m using them both for Thunderbolt (one for the adapter and the other for the drive). I don’t know if any of the USB type A ports support 3.2 or if they are just 3.1. When I tried an external USB3.1 ssd I got a little less speed via the dock compared to directly connected to the iMac (about 320 MB/s vs 350 with a Crucial MX500 1TB in an older cheap USB enclosure).

The TB3 SSD I have is a VisionTek 512GB that came already assembled as an external drive. I don’t know what brand or model the internal NVMe blade is but I assume it’s a lower end unit given the price. It was $99 but looks to be up to $105 now. Just search for VisionTek Thunderbolt SSD on Amazon. They also have a 1TB model for $180 (wasn’t available when I bought mine).

 

[Retired2020]

The dock is HP model HSTNN-CX01. This model is a flat slim brick with several USB ports and two Displayports (haven’t tried DP but USB works). You can find them on eBay for about $25 without the power adapter or about $50 with (power adapters alone are around $15-$50 as well so need to shop around a bit). Mine has a 150W power adapter but they have a 65W model as well that should be fine and might be cheaper.

HP also makes a bigger cubes shaped dock which seems to sell for a bit more. I haven’t tried that one but think it will work as well. That model has what appears to be an attached TB3 cable but the instructions say it can be removed although I’m not sure how much room there is for other TB3 plugs (the connector is under a cover). This would be fine if your TB3 Drive has a detachable cable but mine doesn’t. The Apple TB adapter has a TB3 connector so you need to have a female socket to connect it.

I haven’t tried USB-C (or 3.2). My dock has two C style sockets but I’m using them both for Thunderbolt (one for the adapter and the other for the drive). I don’t know if any of the USB type A ports support 3.2 or if they are just 3.1. When I tried an external USB3.1 ssd I got a little less speed via the dock compared to directly connected to the iMac (about 320 MB/s vs 350 with a Crucial MX500 1TB in an older cheap USB enclosure).

The TB3 SSD I have is a VisionTek 512GB that came already assembled as an external drive. I don’t know what brand or model the internal NVMe blade is but I assume it’s a lower end unit given the price. It was $99 but looks to be up to $105 now. Just search for VisionTek Thunderbolt SSD on Amazon. They also have a 1TB model for $180 (wasn’t available when I bought mine).

I purchased the HSTNN-CX01 and 65w power supply. But when I attach the dock to my late 2015 iMac (OS Catalina) as you explained with the TB cable and adapter, finder doesn't show anything that I've plugged into the USB ports. Checked System Info for the TB bus, which shows the HP TB 3 Dock is connected but that it is unsupported. How did you get your dock to work?

 

[Bremse]

I purchased the HSTNN-CX01 and 65w power supply. But when I attach the dock to my late 2015 iMac (OS Catalina) as you explained with the TB cable and adapter, finder doesn't show anything that I've plugged into the USB ports. Checked System Info for the TB bus, which shows the HP TB 3 Dock is connected but that it is unsupported. How did you get your dock to work?

I didn’t do anything special. Just plugged it in and it worked. I have since switched to a Caldigit TS3+ dock. It seems to run both the dock and the SSD a lot cooler and has some other perks like a front SD slot but the HP dock worked fine and the regular USB ports on it worked as well. No special driver needed. Maybe your dock, TB2/3 adapter, or cable is defective? What port are you using on the dock for the adapter and which one for the SSD? Is your SSD TB3 or USB-C (connectors look the same)?

 

[Retired2020]

Don't believe it is a hardware issue as the System Information with the Thunderbolt Device Tree shows a connected HP Thunderbolt 3 Dock - Unsupported. So it appears to be a software block. It even shows a firmware version 16.1. I've found other websites that say there is a kernel extension hack can be used. I'm using the SSD connected to one of the USB 3.0 ports.

 

[Bremse]

I didn’t do anything special. Just plugged it in and it worked. I have since switched to a Caldigit TS3+ dock. It seems to run both the dock and the SSD a lot cooler and has some other perks like a front SD slot but the HP dock worked fine and the regular USB ports on it worked as well. No special driver needed. Maybe your dock, TB2/3 adapter, or cable is defective? What port are you using on the dock for the adapter and which one for the SSD? Is your SSD TB3 or USB-C (connectors look the same)?

Good point about it not being a hardware issue. I’ll have dig out my HP dock and see what firmware it has. If you’re just using it for the USB3 port though you are not really gaining anything from the dock. Couldn’t you just directly connect the SSD to the iMac’s built in USB3 port?
Edit: I just checked and I have version 16.1 and it says unsupported after the dock name as well. I just tried a USB3 case with a 2.5” SATA SSD inside on several of the regular USB ports on the dock and it was recognized just fine. I don’t have a real USB-C device to try other than an iPhone 7+ with a lightning to USB-C cable and that also works (but again not a real -c or USB3.2 device). The iMac is running Catalina 10.15.5.
Take a look in system profile under USB and see if it shows a USB3.0 Bus entry with HP Dock Audio and ASM107x children. The thunderbolt section in system profiler won’t show anything connected to the dock other than more thunderbolt devices (if you had any connected). Your USB drive should show up in the USB section. Also, Finder will only show the drive if it’s properly formatted. You can also check in disk utility to see if anything shows there.

 

[Retired2020]

Good point about it not being a hardware issue. I’ll have dig out my HP dock and see what firmware it has. If you’re just using it for the USB3 port though you are not really gaining anything from the dock. Couldn’t you just directly connect the SSD to the iMac’s built in USB3 port?
Edit: I just checked and I have version 16.1 and it says unsupported after the dock name as well. I just tried a USB3 case with a 2.5” SATA SSD inside on several of the regular USB ports on the dock and it was recognized just fine. I don’t have a real USB-C device to try other than an iPhone 7+ with a lightning to USB-C cable and that also works (but again not a real -c or USB3.2 device). The iMac is running Catalina 10.15.5.
Take a look in system profile under USB and see if it shows a USB3.0 Bus entry with HP Dock Audio and ASM107x children. The thunderbolt section in system profiler won’t show anything connected to the dock other than more thunderbolt devices (if you had any connected). Your USB drive should show up in the USB section. Also, Finder will only show the drive if it’s properly formatted. You can also check in disk utility to see if anything shows there.

I'm using the external SSD as my boot drive, connected to one of the iMac USB 3.0 ports. It has much faster performance than the the internal HDD. I was hoping to find a reasonable cost TB2/3 boot drive solution. The unsupported HP TB3 dock doesn't pass through anything. It's frustrating. Looks as though the only difference is your 150 w vs. 65 w power supply to the HP dock station.

 

[FriedApples]

Thanks to Bremse's posts above I was able to get a similar setup working on a 27" late 2015 iMac running 10.15.17 from a tiny internal 250GB SSD. Back when I purchased the iMac I figured I would eventually be able to add a larger 3rd party SSD inside the iMac, but presently that is still impossible. I bought the same HP dock that Bremse used from eBay with a new HP 65W power supply, also from eBay. I paired those with an Apple Thunderbolt 2-to-3 adapter and a 2m Apple Thunderbolt 2 cable. I then put a 2TB ADATA XPG SX8200 Pro inside an OWC Envoy Express Thunderbolt 3 enclosure. The SSD was detected immediately and I formatted it in Disk Utility without issue. My rough calculations indicate read/write speeds of at least 1GB/s. I've not thoroughly tested all the ports on the dock, but the NIC and devices attached to any of the other USB ports are at least recognized by macOS. It would be interesting to see if link aggregation is possible with this setup, however, I don't currently own an LACP-capable switch.

Thanks again to Bremse! I'm so glad to both be able to breathe new life into this iMac and readily share data between it and my new M1 Mac Mini.

Edit: an SSD died after attempting to transfer ~300GB from the iMac's internal SSD to the SSD in the enclosure. The dock and/or dock power supply seems to be at fault. The SSD did not overheat, there was no audible sound when the SSD died, but the inside of the enclosure and the SSD smelled fried and macOS no longer recognizes the SSD. The enclosure still works with another 2TB SSD when plugged directly into the Mac Mini. All this happened after a few weeks of having the dock connected to the iMac and it seemingly working fine with non-Thunderbolt devices during that time. I'll likely pick up the Caldigit TS3+ as a replacement for the HP dock.

Edit 2: I purchased a new Caldigit TS3+ dock, received a replacement for the ADATA SSD, and purchased a cheap Inland 1TB 2280 SSD. While I have been successfully testing the Inland 1TB SSD with the enclosure and dock for a few days, the replacement ADATA SSD died again inside the enclosure while attached to the Caldigit dock after the first attempt at writing about 100GB. To be clear, there are videos on YouTube demonstrating that this particular ADATA SSD works with the OWC enclosure and Thunderbolt 3, but apparently something goes horribly awry when it is used in this configuration with Thunderbolt 2.

 

[mlgm]

Hi, I have a Late 2015 27" iMac and I want to use an external SSD to speed up my computer's performance. I was thinking of using a Samsung T5 since I have a few of those and they seem to work really well. Alternatively, I could use a Samsung 860 EVO SSD with a SATA III to USB adaptor. Which is better? They're all around the same price so just wondering if one is more reliable than the other, or more suitable for running an Operating System as well as running applications on a daily basis. I'm really glad I found this discussion thread since I thought I would have to give up on replacing the internal HDD since it was so expensive to get it done by a repair shop and I didn't want to risk breaking my computer by installing an internal SSD myself. Thanks!

 

[baronmax]

@Bremse @FriedApples I also saw Bremse's setup with interest (thanks!) I got the same HP TB3 dock for £25 here in the UK and a 65W HP power supply for another £14 off ebay. I also got a SABRENT ROCKET XTRM Q 512GB for £140 with TB3 connection. I then picked up an Apple TB2/TB3 adaptor and TB2 to TB2 cable off Ebay for £50. Interestingly the HP Dock works fine on my new Macbook Air M1 and sees the SSD, the dock even provides power to the Macbook. HOWEVER on my iMac late 2015 it is shown as 'unsupported' and the hard drive doesn't show up. Same on my old 2015 Macbook Pro running Mavericks. Bit of a mystery how you both got it to work, although FriedApples didn't seem to have a good experience. My HP Elite dock has firmware version 18.1; it can be updated but would need an intel computer running windows with a TB3 port. NB both the iMac and Macbook air M1 are running Big Sur. There is a hack on github that enables unsupported TB3 docks but whilst trying to do this I typed a terminal command in wrong which screwed up disk permissions and meant I was forced to do a fresh install of Big Sur => result the iMac now runs faster (!) with its 1TB fusion drive (half full) - The fastest I'm getting is write/read of 390/859 MB/s using Blackmagic 1GB setting whereas before I was getting about 368/469 as the best reading and quite often a lot lower than that.

Anyway the hack didn't seem the best option as you have to turn off system protection so it can load something all the time. So I got another TB3 dock, an Alogic one for £55 and this one seems to work a treat - am getting almost the same speed on this if I directly connected it to the Macbook Air M1 - write/read of 810/1260 (I think the M1 had a slightly higher read speed of 1600). Not sure why the write is slower maybe this is a Black magic thing or maybe the SABRENT ROCKET isn't as fast as other SSDs for some reason. Its formatted as HFS+.

Next step is to install Big sur on the ROCKET or clone the Fusion disk (well <512GB of it) and then use that as my boot drive for the iMac... SO in summary a) using the HP dock could be good but everyone here seems to have rejected it as a long term solution b) there's a fair bit of cost in getting a TB3/SSD solution but hopefully should extend the life of my iMac for a good few years if successful, plus I didn't have the stress of trying to open it up and fit one inside.

 

[baronmax]

Hi, I have a Late 2015 27" iMac and I want to use an external SSD to speed up my computer's performance. I was thinking of using a Samsung T5 since I have a few of those and they seem to work really well. Alternatively, I could use a Samsung 860 EVO SSD with a SATA III to USB adaptor. Which is better? They're all around the same price so just wondering if one is more reliable than the other, or more suitable for running an Operating System as well as running applications on a daily basis. I'm really glad I found this discussion thread since I thought I would have to give up on replacing the internal HDD since it was so expensive to get it done by a repair shop and I didn't want to risk breaking my computer by installing an internal SSD myself. Thanks!

Ideally you'll want what I did above for maximum performance (using use Thunderbolt). Using USB the speed will be limited but I don't have a decent USB-C lead to SSD lead otherwise I could tell you by how much. I think others still think its good, so you could try first, then if not good get a SSD to TB caddy and put the SSD board in that...

 

[Bremse]

Thanks to Bremse's posts above I was able to get a similar setup working on a 27" late 2015 iMac running 10.15.17 from a tiny internal 250GB SSD. Back when I purchased the iMac I figured I would eventually be able to add a larger 3rd party SSD inside the iMac, but presently that is still impossible. I bought the same HP dock that Bremse used from eBay with a new HP 65W power supply, also from eBay. I paired those with an Apple Thunderbolt 2-to-3 adapter and a 2m Apple Thunderbolt 2 cable. I then put a 2TB ADATA XPG SX8200 Pro inside an OWC Envoy Express Thunderbolt 3 enclosure. The SSD was detected immediately and I formatted it in Disk Utility without issue. My rough calculations indicate read/write speeds of at least 1GB/s. I've not thoroughly tested all the ports on the dock, but the NIC and devices attached to any of the other USB ports are at least recognized by macOS. It would be interesting to see if link aggregation is possible with this setup, however, I don't currently own an LACP-capable switch.

Thanks again to Bremse! I'm so glad to both be able to breathe new life into this iMac and readily share data between it and my new M1 Mac Mini.

Edit: an SSD died after attempting to transfer ~300GB from the iMac's internal SSD to the SSD in the enclosure. The dock and/or dock power supply seems to be at fault. The SSD did not overheat, there was no audible sound when the SSD died, but the inside of the enclosure and the SSD smelled fried and macOS no longer recognizes the SSD. The enclosure still works with another 2TB SSD when plugged directly into the Mac Mini. All this happened after a few weeks of having the dock connected to the iMac and it seemingly working fine with non-Thunderbolt devices during that time. I'll likely pick up the Caldigit TS3+ as a replacement for the HP dock.

Edit 2: I purchased a new Caldigit TS3+ dock, received a replacement for the ADATA SSD, and purchased a cheap Inland 1TB 2280 SSD. While I have been successfully testing the Inland 1TB SSD with the enclosure and dock for a few days, the replacement ADATA SSD died again inside the enclosure while attached to the Caldigit dock after the first attempt at writing about 100GB. To be clear, there are videos on YouTube demonstrating that this particular ADATA SSD works with the OWC enclosure and Thunderbolt 3, but apparently something goes horribly awry when it is used in this configuration with Thunderbolt 2.

I wonder if it’s a thermal issue with this particular SSD in this particular enclosure. Maybe the reduced transfer rate of running at TB2 cause the transfer to take longer. Certainly interesting. I have been running with the Caldigit dock for a while now and use the iMac every day without issue. The with the caldigit dock the SSD seems a lot cooler than with the HP (and the dock runs cooler itself as well).

 

[Bremse]

@Bremse @FriedApples I also saw Bremse's setup with interest (thanks!) I got the same HP TB3 dock for £25 here in the UK and a 65W HP power supply for another £14 off ebay. I also got a SABRENT ROCKET XTRM Q 512GB for £140 with TB3 connection. I then picked up an Apple TB2/TB3 adaptor and TB2 to TB2 cable off Ebay for £50. Interestingly the HP Dock works fine on my new Macbook Air M1 and sees the SSD, the dock even provides power to the Macbook. HOWEVER on my iMac late 2015 it is shown as 'unsupported' and the hard drive doesn't show up. Same on my old 2015 Macbook Pro running Mavericks. Bit of a mystery how you both got it to work, although FriedApples didn't seem to have a good experience. My HP Elite dock has firmware version 18.1; it can be updated but would need an intel computer running windows with a TB3 port. NB both the iMac and Macbook air M1 are running Big Sur. There is a hack on github that enables unsupported TB3 docks but whilst trying to do this I typed a terminal command in wrong which screwed up disk permissions and meant I was forced to do a fresh install of Big Sur => result the iMac now runs faster (!) with its 1TB fusion drive (half full) - The fastest I'm getting is write/read of 390/859 MB/s using Blackmagic 1GB setting whereas before I was getting about 368/469 as the best reading and quite often a lot lower than that.

Anyway the hack didn't seem the best option as you have to turn off system protection so it can load something all the time. So I got another TB3 dock, an Alogic one for £55 and this one seems to work a treat - am getting almost the same speed on this if I directly connected it to the Macbook Air M1 - write/read of 810/1260 (I think the M1 had a slightly higher read speed of 1600). Not sure why the write is slower maybe this is a Black magic thing or maybe the SABRENT ROCKET isn't as fast as other SSDs for some reason. Its formatted as HFS+.

Next step is to install Big sur on the ROCKET or clone the Fusion disk (well <512GB of it) and then use that as my boot drive for the iMac... SO in summary a) using the HP dock could be good but everyone here seems to have rejected it as a long term solution b) there's a fair bit of cost in getting a TB3/SSD solution but hopefully should extend the life of my iMac for a good few years if successful, plus I didn't have the stress of trying to open it up and fit one inside.

I wonder if it’s an issue with NVMe drivers. The computers boot rom needs to be up to date to use external NVMe drives. I think you have to install Catalina (or maybe high Sierra works as well - not sure on that) on that machine from a non-NVMe drive first. This will update the boot ROM. After that you should be able to use external NVMe drives. My wife uses a 2016 16” touchbar MBP with Sierra and it does not recognize the external NVMe SSD because it has not had the boot ROM updated. Also note that the HP dock shows unsupported for me as well but appears to work anyways.