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Fooling a mac to believe that external disk is internal?

T

toke lahti

macrumors 68040
Original poster
Apr 23, 2007
3,293
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Helsinki, Finland
I have a 2018mini with 256GB of internal storage.
I'd like to use that to make a 2-4 TB fusion drive with external hdd.

Is it possible?

Background:
I took a internal DIY fusion drive out from my 2012mini to inspection and the fusion drive does mount as both drives external.
But it disappears in few minutes since the Sandisk ssd is flakey and need to be replaced...
 
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You DO NOT want to do this.

You are going to make that [relatively] fast internal drive GO SLOWER.
Why do that?

And further... a fusion drive (particularly one created with an external drive) is more "fragile" in nature than is a drive "that exists unto itself" -- because it's relying on external software (like the Mac OS). Just have a mishap where the external gets jostled or disconnected... and it could damage the whole of the "fusion" drive (and file system).

Why not just keep TWO drive icons on the desktop?
You'll know where everything's supposed to go...
 
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Fishrrman said:
You DO NOT want to do this.

You are going to make that [relatively] fast internal drive GO SLOWER.
Why do that?

And further... a fusion drive (particularly one created with an external drive) is more "fragile" in nature than is a drive "that exists unto itself" -- because it's relying on external software (like the Mac OS). Just have a mishap where the external gets jostled or disconnected... and it could damage the whole of the "fusion" drive (and file system).

Why not just keep TWO drive icons on the desktop?
You'll know where everything's supposed to go...
Click to expand...
Why?
Because I like the idea of having fast and cheap storage.
Anyways, I can't use that internal storage to anything else.

Can you explain why the internal ssd would work slower and how much?

I've had now 3 fusion drives, where ssd has failed. Since Apple's ssd is so much better (hence the 9x price), it would be optimal for fusion drive.

I don't care if it's fragile and I will believe the fragility only with real filed testing.
I can always boot from my external ssd as I have done for the last 3 years.

"Two icons" is pretty wild oversimplification.
My /Apps is 108GB and
my ~/Lib is 251GB.

Do You want to sort them out to "two icons"?

Btw, can Monterey still be installed to hfs+?
I suppose there's no fusion drive with apfs?
 
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toke lahti said:
I suppose there's no fusion drive with apfs?
Click to expand...
Sure there are. My iMac (2019) has a fusion drive and it is APFS.

As to the fragility of such a setup, make sure to have a really good backup if you do this. My assumption is that anything that is externally connected could be unplugged at any time.
 
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There are methods to create your own logical core storage volume merging two physical drives, but does that actually get you the "fusion drive" magic of automatically tiering into hot and cold storage? Nowhere in the instructions do I see where you specify one drive as hot storage and the other as cold storage, so how could osx possibly know which is the faster one?

~~Maybe this is somehow automatically inferred based on transfer speed or something, since the official apple instructions to recombine a fusion drive don't do anything special either https://support.apple.com/en-us/HT207584~~ No actually I think the key is the order you specify things in the createVolume command. The SSD should come first. C.f. https://eshop.macsales.com/blog/36227-everything-you-need-to-create-your-own-fusion-drive/ and https://apple.stackexchange.com/que...onversion-to-apfs-from-hfs-fusion-drive-fails
 
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toke lahti said:
Can you explain why the internal ssd would work slower and how much?
Click to expand...
Because you will create a bottleneck. The internal SSD is much faster than the external SSD so it will have to wait for new data transfer request to the slower external drive. It’s like buying a Chevrolet Corvette for the sole purpose of driving it in school zones.
 
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Heat_Fan89 said:
Because you will create a bottleneck. The internal SSD is much faster than the external SSD so it will have to wait for new data transfer request to the slower external drive. It’s like buying a Chevrolet Corvette for the sole purpose of driving it in school zones.
Click to expand...
Nope,
all data that is needed either fast or frequently lies in fast ssd.
That is the whole idea of fusion drive.

Making a fusion drive out of 250GB ssd and 2-4TB hdd, would mean that for me, all frequently needed stuff will be in ssd side. Music/videos/photos/movies in slow side.
Without having to manually organizing them.

I'm not sure how fusion drive internally works, but I guess, that organizing the files happen at the background and there's no queries when file is asked to open; either it will open from ssd or there is a hard link to hdd side.
Am I guessing right?

So the bottleneck exists only when speed is not needed.

Fusion drive was the second reason I kept using macos. First was time machine. Neither one was offered by the competitor. As much I would have liked to build me fast but cheap hardware.
 
curnalpanic said:
Sure there are. My iMac (2019) has a fusion drive and it is APFS.

As to the fragility of such a setup, make sure to have a really good backup if you do this. My assumption is that anything that is externally connected could be unplugged at any time.
Click to expand...
Okay!
Seems pretty straightforward, back in 2019, when apfs was young, IIRC, you couldn't do fusion with apfs and with hfs+ disks, the answer was "only with internal disks".

I call this progress!
(Which no apple employee will ever tell to customer.)

For the past 2 years (moving from Mojave to Monterey) I have had soooo many problems with TM that I have doubled down my backup routines, so they are fine.
In emergencies, I will have external Mojave install to boot, external Monterey install to boot and I guess, that I could slice 50GB from internal drive to "local boot" without fusion drive.

With external Mojave there was lot of issues for repairing permissions and local Apple support refused to help, if I tried to repair permissions of any disk that doesn't have macos to boot with. I then created a habit, that I install macos to every external drive that I use frequently. So I have a lot of "ready to boot" installs.
The infamous non-documented command that did not work for external drives was: " diskutil resetUserPermissions / `id -u` ", and to be precise, the switch " `id -u` ".
Support newer goth the anser from engineers that is there a way to use that switch with other than internal drive.

I miss the days of cMP, when I could swap in and out multiple internal storages, but I do not miss the electricity bill...
 
Damn Apple!
You archive documentation and then change things…

About Fusion Drive, a storage option for some Mac computers - Apple Support

Fusion Drive combines the performance of Flash storage with the capacity of a hard drive.
support.apple.com support.apple.com
”
  • An external drive can't be used as part of a Fusion Drive volume.”


I still haven’t got time to try this…

what is the cheapest HDD case using TB?
 
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Wow!
Lots of documentation on man pages of diskutil for APFS fusion drive:

Code: 
createContainer [-main] device [-secondary] [device]
                           Create an empty APFS Container.  The device(s) specified become APFS Physical Stores.  If you specify more than
                           one device, a Fusion Container is created.

                           For Fusion cases, if you do not explicitly use the -main and -secondary options, the performance duties are
                           assigned automatically; this is preferred.  Rotational vs. solid-state hardware design must be detectable; this
                           is often not the case for external disks. Solid-state hardware is welcome but not required; it is the
                           identification which holds as a hard requirement with this usage.

                           Alternatively, you can explicitly specify -main and -secondary devices; if you do so, you must specify both.  The
                           "main" device is assumed to be "faster" (you should use solid-state hardware if available), while the "secondary"
                           device is assumed to be "slower" and is often used to store OS-associated "auxiliary" data such as a Boot Camp
                           Assistant partition.

Seems to be that things have secretely evolved!
Anybody ever heard about this progress?

Maybe, some day, we will have 2 ram levels in macs: fast on-chip non-upradeable and slower upgradeable on mb...
 
Bigwaff said:
It is possible. This bloke did it, however, the SSD was external is his case.
artspb.me

Using external SSD as macOS Fusion Drive

The post describes how to configure macOS Fusion Drive using an external SSD.
artspb.me artspb.me

I empathize with your situation. Like, @curnalpanic , I recommend having really frequent backups.
Click to expand...
I have to give up. Tried different combinations all night. Making the Volumes in Disk Utility and then pointing to containers or synthesized volumes, always the same:
Code: 
Error: -69614: The disk is already in use by APFS
Deleting the synthesized volumes and same result.
 
Sadly OS does not install to external Fusion.
Tried FROM both Ventura & Monterey, to install both Ventura and Monterey.
Same result:
1777954E-1CF1-4B5C-9CE4-B1689B58510C.jpeg
 
Tried to do this with Catalina installation usb-stick, since Artem, who wrote about successful "external fusion" used that.

This time (Catalina's) diskutil refuses to fusion drives, because my mini2018 recognises all my ssd's as not "solid state".
Tried with all my adaptors and enclosures.

Any way around this bug?
Are there certain adapters or ssd's that are reported correctly?
 
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toke lahti said:
Wrapup: as soon as I found a hdd/ssd case that truly supported UASP (all my Orico cases are said to support, but none does), creating external fusion had no problems whatsoever.

I noticed fun fact. Slower part of Fusion drive needs to be bigger than faster drive. Tried with 480GB ssd and 320/500GB hdd's.
Click to expand...
Hi,

My 2013 iMac is dead so I would like took out the fusion drive, hoping to get the files in it.
I put it into SATA external drive enclosure but cannot read the drive except for the BootCamp partition.

You look done all this hassles so I would like to get advice especially about your comment
"The volumes to be Fusioned has to be first in hfs+!"

This fusion drive was used in iMac with 10.15 Catalina so it should be APFS.
Now I am trying to mount it in MacbookAir with 12.6 Monterey.

This is the result of
> diskutil list
/dev/disk2 (external, physical):
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *3.0 TB disk2
1: EFI ⁨EFI⁩ 209.7 MB disk2s1
2: Apple_APFS ⁨⁩ 2.5 TB disk2s2
3: Microsoft Basic Data ⁨BOOTCAMP⁩ 499.8 GB disk2s3
4: Windows Recovery ⁨⁩ 550.5 MB disk2s4

I do see the HDD portion but SSD doesn't show up so I cannot set "RAID Span of Storage" to restore them.
Creating container failes too.

> diskutil apfs createContainer /dev/disk2s2
Creating container with disk2s2
Started APFS operation on disk2s2
Creating a new empty APFS Container
Error: -69614: The disk is already in use by APFS
 
Yes and no: No fusion.

I boot from an external drive enclosere Acasis 40Gbps M.2 Nvme SSD Enclosure with WD_BLACK SN850X NVMe SSD 2 TB.

Just install a fresh copy of Mac OS and migrate your stuff to the external drive.

It's way faster than the internal drive(2700+ mb/s for both read/write).
 
SSDs are so cheap at this point that there is really no reason to consider a Fusion Drive as a solution...
 
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