Help with target disk mode on 2015 iMac 5k?

[phineas_gage]

Hi all,
Thanks for any help you can provide--I have a late 2015 iMac 5k Core i7 with a 2TB fusion drive.
I tried installing the 11.7.7 update a week back and it did not go well--hanging on boot, occasionally would fully boot but running very slow (re-indexing the drive?). Booting into safe mode and running Disk Utility did not report errors, but also didn't fix the problem. Eventually it stopped booting all together, and booting from a Thumb Drive and trying to re-install failed due to an inability to mount the drive.
I'm trying to boot into target disk mode to see if I can mount the drive that way and either re-install or move files. I stupidly did not back up before this update because I was careless and have learned my lesson.
My questions are:
1) Is this a viable way to fix this (through trying target disk mode?)
2) From what I've read, it will only boot into target disk mode through the Thunderbolt 2 port--I don't have a Thunderbolt 2 cable (yet). The other Macs I'm trying to use to connect to it are all Thunderbolt 3. Will this work through a dongle/adapter? Meaning, I get a Thunderbolt 2 cable and adapter, plug it into my other Mac. Or, does anyone know of a cable which is Thundebolt2 to Thunderbolt3 that will work for this?

Thanks for any and all help!

 

[theMarble]

Thanks for any help you can provide--I have a late 2015 iMac 5k Core i7 with a 2TB fusion drive.
I tried installing the 11.7.7 update a week back and it did not go well--hanging on boot, occasionally would fully boot but running very slow (re-indexing the drive?). Booting into safe mode and running Disk Utility did not report errors, but also didn't fix the problem. Eventually it stopped booting all together, and booting from a Thumb Drive and trying to re-install failed due to an inability to mount the drive.

Those are nearly all of the signs of a dying/nearly dead hard drive. Keep in mind that while the Fusion Drives are slightly faster because of the SSD cache, they are still spinning rust at the end of the day.

1) Is this a viable way to fix this (through trying target disk mode?)
2) From what I've read, it will only boot into target disk mode through the Thunderbolt 2 port--I don't have a Thunderbolt 2 cable (yet). The other Macs I'm trying to use to connect to it are all Thunderbolt 3. Will this work through a dongle/adapter? Meaning, I get a Thunderbolt 2 cable and adapter, plug it into my other Mac. Or, does anyone know of a cable which is Thundebolt2 to Thunderbolt3 that will work for this?

1. Yes, that would work, however in my personal opinion, I would get all off the data off and get an SSD. I would never touch that hard drive / fusion drive anymore.

2. Apple sells a Thunderbolt 3 to 2 adapter. Hook the TB3 end into another Mac, and get a TB2 cable (which Apple also sell) and hook that end into the iMac.

 

[Amethyst1]

The 2TB Fusion Drive includes a 128GB PCIe SSD. It’s possible the SSD has gone bad but it pretty much sounds like the HDD is the culprit. If the small SSD is still good it might be useful for something.

 

[theMarble]

The 2TB Fusion Drive includes a 128GB PCIe SSD. It’s possible the SSD has gone bad but it pretty much sounds like the HDD is the culprit. If the small SSD is still good it might be useful for something.

How often is it that the SSD in a Fusion Drive fails before the HDD? I know NAND does die eventually, but it seems a lot more likely that it would be the HDD part.

 

[Amethyst1]

Yeah. What I was trying to say is if the 128GB PCIe SSD is still good it may be useful for something, maybe as an "accelerator" for the SATA SSD that's replacing the HDD, i.e. in another Fusion Drive setup. Or use the PCIe SSD for the OS and apps and the SATA SSD for data. Or...

 

[phineas_gage]

Yeah. What I was trying to say is if the 128GB PCIe SSD is still good it may be useful for something, maybe as an "accelerator" for the SATA SSD that's replacing the HDD, i.e. in another Fusion Drive setup. Or use the PCIe SSD for the OS and apps and the SATA SSD for data. Or...

Thanks to you both. It’s been a great machine and I still use it all the time. I saw that the option to use it in target display mode was killed after one of the OS updates, and I’ll look this up also, but if I can drop an SSD in this to replace the Fusion Drive, I may do that

 

[Amethyst1]

I saw that the option to use it in target display mode was killed after one of the OS updates, […]

Target Display Mode has never worked with 4K/5K iMacs. It’s not the same as Target Disk Mode, which is what allows accessing the iMac’s drives from another system.

 

[phineas_gage]

Target Display Mode has never worked with 4K/5K iMacs. It’s not the same as Target Disk Mode, which is what allows accessing the iMac’s drives from another system.

Right—what I meant was that the screen still looks great and instead of scrapping the working parts because the thing that was most likely to fail first….failed first, i would love to use it as a display. But, not going to happen.
I don’t know that I’m interested in tracking down a replacement Fusion Drive, but putting an SSD in it may give this thing a new lease on life.

 

[Amethyst1]

[…] i would love to use it as a display. But, not going to happen.

DIY 5k Monitor - success :-)

My "latest Macs" are a heavily upgraded 2009 Mac Pro (6c CPU, M.2 SSD, USB 3, RX580...) and more recently a 12c 2013 Mac Pro with an eGPU (Vega 56) - both machines with more than enough CPU power, GPU power and RAM for my needs, but I long aspired to upgrade to a nice 5k monitor to replace my...

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