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How to set up a dual SSD Fusion Drive?

I own a 27-inch late 2012 iMac. The HDD in my 3 TB Fusion Drive has died, so I replaced it with a 2 TB SATA-SSD. Since the 128 GB M.2 Apple SSD is probably still a lot faster than the new SSD, I wanted the two drives to operate in "fusion mode" once again, rather than have two separate volumes.

Using diskutil, I unmounted the Fusion Drive, erased each of the physical disks, created a new CoreStorage Fusion Group (diskutil cs create "Macintosh HD" disk0 disk1) and then a CoreStorage Fusion volume (diskutil cs createVolume "Macintosh HD" JHFS+ "Macintosh HD" 100%). I then installed macOS High Sierra with no issues and migrated my data from a Time Machine backup. Everything appeared to be just fine until I arrived at installing Windows: Boot Camp would only let me use 110 GB for the new partition despite one TB of free disk space.

Another look at the drive configuration revealed that something has gone terribly wrong after all:

/dev/disk0 (internal, physical):

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme *121.3 GB disk0

1: EFI EFI 209.7 MB disk0s1

2: Apple_CoreStorage Macintosh HD 24.5 GB disk0s2

3: Apple_Boot Recovery HD 650.0 MB disk0s3

4: Microsoft Basic Data BOOTCAMP 96.0 GB disk0s4

/dev/disk1 (internal, physical):

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme *2.0 TB disk1

1: EFI EFI 209.7 MB disk1s1

2: Apple_CoreStorage Macintosh HD 2.0 TB disk1s2

3: Apple_Boot Boot OS X 134.2 MB disk1s3

/dev/disk2 (internal, virtual):

#: TYPE NAME SIZE IDENTIFIER

0: Apple_HFS Macintosh HD +2.0 TB disk2

Logical Volume on disk0s2, disk1s2

EAC5E1EA-BD4E-4295-992D-807629B3BECC

Unencrypted

The Boot Camp partition has been created on the Apple SSD while the SATA SSD appears to be the boot volume! Windows located on the Apple SSD is particularly unexpected since with the original configuration, Boot Camp could only access the HDD portion of the Fusion Drive.

Starting over from scratch, how can I create a Fusion Drive with the two SSDs that behaves like it is supposed to?

macOS and frequently used files are stored on (or dynamically moved to) the Apple SSD.
All other files and the Boot Camp partition go to the SATA SSD (just like they did before with the HDD).

Thank you!

iMac, macOS High Sierra (10.13.3)

Posted on Mar 14, 2018 3:44 PM

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23 replies

Kappy

Level 10

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Mar 15, 2018 9:39 AM in response to Portico

I am not booted from my Fusion Drive, but About This Mac looks like this:

Portico Author

Level 1

5 points

Mar 15, 2018 10:26 AM in response to Kappy

Thank you, Glen. What does the Storage tab say? The Fusion Drive label is supposed to appear there.

Kappy

Level 10

362,007 points

Mar 15, 2018 11:38 AM in response to Portico

It just says the volume name, Macintosh HD.

I have a 1TB HDD with a small 24GB PCIe flash drive.

Kappy

Level 10

362,007 points

Mar 16, 2018 11:08 AM in response to Portico

I think you are good as long as it shows what it is in diskutil list.

Portico Author

Level 1

5 points

Mar 16, 2018 11:25 AM in response to Kappy

In my case it just says „Unencrypted“. :-(

Kappy

Level 10

362,007 points

Mar 16, 2018 11:44 AM in response to Portico

How about the rest of the listing? Do you see the two lines above that line? The information you followed is an Apple support document so it should work.

Portico Author

Level 1

5 points

Mar 16, 2018 1:07 PM in response to Kappy

This is the last part of the diskutil output:

It looks okay, apart from the missing Fuison Drive bit.

Mar 18, 2018 3:33 AM in response to Portico

Portico wrote:...I‘ve never checked myself but seen benchmarks where M.2 SSDs were twice or up to three times as fast as SATA SSDs....

were those based on the M.2 being connected to the on-board SATA3 (6.0Gb/s) interface in the 2012 iMac? I think the interface for your flash drive is the same speed as the internal drive bay, so even though an M.2 flash drive might be able to perform faster in a different machine with a faster interface, I don't think the iMac M.2 will perform any better than an SSD on the same speed interface built-in to the motherboard. I can't find detailed specs that show the flash drive interface spec is any different from the on-board SATA3 interface that the main hard drive is connected to, I could be totally wrong about that, if someone can post a link to those detailed specs that would be great.

I understand you prefer the simplicity of rebuilding the same Fusion drive you had before upgrading from an RPM drive to an SSD drive. But the Fusion drive scheme was not intended to join two of the same kind of flash drives together that have roughly the same speed. Seems like a lot of effort to create a drive setup that is probably not an optimal use of the drives that you now have. The Fusion drive you are building now will move your data around from one SSD to another for no reason, you will see no real boost in performance and actually decrease the performance and lifetime of the drives.

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How to set up a dual SSD Fusion Drive?