I have an external SSD drive which has 4 bootable partitions holding monthly backups. It has a GUID partition map; each partition is MacOS Extended Journaled (HFS+); each is 250GB in size with about 120GB in use. The partitions are not encrypted; I do not think it uses Core Storage.

/dev/disk2 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk2
   1:                        EFI EFI                     209.7 MB   disk2s1
   2:                  Apple_HFS SSDmon1                 250.0 GB   disk2s2
   3:                  Apple_HFS SSDmon2                 249.9 GB   disk2s3
   4:                  Apple_HFS SSDmon3                 249.6 GB   disk2s4
   5:                  Apple_HFS ARCH_B3                 249.9 GB   disk2s5

I recently upgraded from Mojave 10.14.6 to Catalina 10.15.7 on this 2012 MacBook Pro.

I used SuperDuper! to back up the new system to a different external drive, and it needed to convert the destination to APFS. So I guess bootable backups now need to be APFS.

My questions:

  • Can I convert the entire external SSD to APFS? How?

  • Or do I need to convert each partition separately? (I know I can do that with Disk Utility or 'diskutil')

  • If the second, will they be put into the same container, or will there be four containers with one volume each? If four containers, can they be joined into one container?

I'm afraid to do something wrong and mess up the drive, so I'd like to hear suggestions and warnings before I attempt anything. It would not be the end of the world to lose the backups, but I'd rather avoid that if I could.

  • The simple answer would be convert each JHFS+ formatted partition on the external drive to an APFS container as you do your monthly backups. A more complex answer would be difficult to post without knowing which version of OS X/macOS you upgraded from, whether Core Storage was in use and if encryption is being used. Might also be helpful to post the diskutil list output for the external drive(s). Commented May 30, 2023 at 16:48
  • Ah. I upgraded from Mojave 10.14.6. I do not think Core Storage was used. No encryption. I'll add this to the question. Thanks!
    – aMike
    Commented May 31, 2023 at 12:25
  • I added the diskutil output. I guess I'll just convert them as-needed and see what results. Thanks!
    – aMike
    Commented May 31, 2023 at 12:32

1 Answer 1


I ended up converting the first partition to APFS, then removed the next two partitions, and extended the APFS to recover the newly made space. So now there's one 750GB container, with six volumes inside (system and data for each of three months). I left the fourth partition alone as HFS+, since it did not have an OS on it.

In other words, I couldn't do anything I really wanted to do.

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