How do I split Fusion drive on Mojave / APFS?

Question

I have a 2013 iMac (27" 3.5GHz i7, model id iMac14,2, EMC 2639) that has a Fusion drive. I want to replace the Fusion's internal HDD with an SSD. My understanding is that I should split the Fusion drive before performing this.

I referred to the article https://www.macworld.com/article/2015664... which discusses the use of diskUtil and Core Storage to get the GUIDs and split the drive.

However, invoking disktutil coreStorage list returned ‘No CoreStorage logical volume groups found’. I then tried diskutil list, which returned more information, apparently showing the fused drives as ‘synthesized’. I read a bit about this in other discussions, and it seems like this is perhaps a Mojave/APFS version of a Core Storage Fusion config.

I’m wondering what I need to do to split my Fusion drive. Is there something else to use, or do I not even need to split the HDD/SSD before replacing the HDD?

Here is the output from diskutil list (I'm booted from an external drive named "External Drive". I think the first three entries are for my internal Fusion drive, and the last two entries are for the external drive:

/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *3.0 TB     disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                 Apple_APFS Container disk2         3.0 TB     disk0s2

/dev/disk1 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *121.3 GB   disk1
   1:                        EFI EFI                     209.7 MB   disk1s1
   2:                 Apple_APFS Container disk2         121.1 GB   disk1s2

/dev/disk2 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +3.1 TB     disk2
                                 Physical Stores disk1s2, disk0s2
   1:                APFS Volume Preboot                 28.7 KB    disk2s2
   2:                APFS Volume Recovery                20.5 KB    disk2s3
   3:                APFS Volume VM                      2.1 GB     disk2s4
   4:                APFS Volume Macintosh HD            9.6 MB     disk2s1

/dev/disk3 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *4.0 TB     disk3
   1:                        EFI EFI                     209.7 MB   disk3s1
   2:                 Apple_APFS Container disk4         4.0 TB     disk3s2

/dev/disk4 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +4.0 TB     disk4
                                 Physical Store disk3s2
   1:                APFS Volume External Drive          81.6 GB    disk4s1
   2:                APFS Volume Preboot                 45.7 MB    disk4s2
   3:                APFS Volume Recovery                512.5 MB   disk4s3
   4:                APFS Volume VM                      17.2 GB    disk4s4

Thanks.

-Allan

Answer 1

I did this earlier today on an iMac. Boot to a Mojave USB boot installer. Then Utilities → Terminal. The commands are along the lines of:

diskutil apfs list

Then make note of the “virtual disk” that is the “Fusion Drive”. This is made up the PCI Flash and the other spinning drive. You can see how the text output of the drive structure is hierarchical.

Let’s say that virtual container disk is identified as “disk0”

diskutil apfs deletecontainer disk0

This command will split your fusion drive, and leave you with two new drives to format via Disk Utility (which you can run in the same USB Booted environment you’re already in).

Format them (the SSD and the other drive) as whatever you want.

I recently did this prior to replacing the slow spinning disk that comprised the 1TB of my Fusion Drive. Replaced that with a 500GB SSD which is SATA3. Installed Mojave onto the 24GB PCI “Blade” Flash and formatted the other SSD.

I then store User home folder on the larger SSD, and macOS and other apps run off the faster PCI-based SSD.

Answer 2

You do not need to split the Fusion Drive before replacing one of the components. The act of changing the hardware will 'break' the logical volume, and the OS will then treat the two devices as separate.

You will have to erase/re-partition the remaining device.

Of course, splitting the Fusion Drive in this way will render the data on it inaccessible, so you must have a backup to restore your data to each volume as you wish.