MacOS – Why can I only have 2 partitions on the 3TB Apple Fusion Drive

core-storagedisk-utilityfusion-drivemacospartition

According to Apple, I can only have 2 partitions on my 3 TB Fusion Drive. I do not understand why. Techtool Pro made an eDrive, their emergency boot drive, and Apple made their Recovery HD. It seems to me that I have 4 partitions on my Fusion Drive, something which Apple says cannot happen.

Source:

…

Can I add a partition to the hard disk with Fusion Drive?

Using Disk Utility, you can add one partition to the hard disk on Fusion Drive. >Once you add the partition, the "plus" symbol in Disk Utility to add additional >partitions will be grayed out. You cannot partition the Flash storage.

…

Any explanations?

Best Answer

It's indeed impossible to create a third regular partition in Disk Utility.

The +-button is greyed out (please check the screenshot below):

enter image description here

The superior HDD is my CoreStorage Volume Group, the inferior HDD is my CoreStorage Volume (spanning over disk1s2 and disk2s2) and HDD 2 is a simple JHFS+ non-CoreStorage volume.

The complete volume list and the partition table of disk1 looks like this:

root# diskutil list
/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *3.0 TB     disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                  Apple_HFS test1                   1.5 TB     disk0s2
   3:                  Apple_HFS test2                   1.5 TB     disk0s3
/dev/disk1
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *68.7 GB    disk1
   1:                        EFI EFI                     209.7 MB   disk1s1
   2:          Apple_CoreStorage                         33.1 GB    disk1s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk1s5
   4:                  Apple_HFS HDD 2                   34.7 GB    disk1s4
/dev/disk2
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *17.2 GB    disk2
   1:                        EFI EFI                     209.7 MB   disk2s1
   2:          Apple_CoreStorage                         16.8 GB    disk2s2
   3:                 Apple_Boot Boot OS X               134.2 MB   disk2s3
/dev/disk3
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                  Apple_HFS HDD                    *47.3 GB    disk3

root# gpt -r -vv show /dev/disk1
gpt show: /dev/disk1: mediasize=68719476736; sectorsize=512; blocks=134217728
gpt show: /dev/disk1: PMBR at sector 0
gpt show: /dev/disk1: Pri GPT at sector 1
gpt show: /dev/disk1: Sec GPT at sector 134217727
      start       size  index  contents
          0          1         PMBR
          1          1         Pri GPT header
          2         32         Pri GPT table
         34          6         
         40     409600      1  GPT part - C12A7328-F81F-11D2-BA4B-00A0C93EC93B
     409640   64562272      2  GPT part - 53746F72-6167-11AA-AA11-00306543ECAC
   64971912    1269536      3  GPT part - 426F6F74-0000-11AA-AA11-00306543ECAC
   66241448   67714096      4  GPT part - 48465300-0000-11AA-AA11-00306543ECAC
  133955544     262151         
  134217695         32         Sec GPT table
  134217727          1         Sec GPT header

After booting to another boot disk and deleting HDD 2 (disk1s4), I added two new partitions (hdd2 and hdd3) instead with gpt add...(check screenshot below)

enter image description here

The partition table of the same disk (the DiskIdentifier changed meanwhile from disk1 to disk0):

root$ gpt -r -vv show /dev/disk0
Password:
gpt show: /dev/disk0: mediasize=68719476736; sectorsize=512; blocks=134217728
gpt show: /dev/disk0: PMBR at sector 0
gpt show: /dev/disk0: Pri GPT at sector 1
gpt show: /dev/disk0: Sec GPT at sector 134217727
      start       size  index  contents
          0          1         PMBR
          1          1         Pri GPT header
          2         32         Pri GPT table
         34          6         
         40     409600      1  GPT part - C12A7328-F81F-11D2-BA4B-00A0C93EC93B
     409640   64562272      2  GPT part - 53746F72-6167-11AA-AA11-00306543ECAC
   64971912    1269536      3  GPT part - 426F6F74-0000-11AA-AA11-00306543ECAC
   66241448     158552         
   66400000   30000000      4  GPT part - 48465300-0000-11AA-AA11-00306543ECAC
   96400000     200000         
   96600000   30000000      5  GPT part - 48465300-0000-11AA-AA11-00306543ECAC
  126600000    7617695         
  134217695         32         Sec GPT table
  134217727          1         Sec GPT header

Result:

Apple is right: you can't add a third volume with Disk Utility. But it's no problem to add a second and a third (and probably even some more) regular JHFS+ volume from the command line booted from another boot disk.

The reason why Apple doesn't allow to add more volumes in Disk Utility is beyond my knowledge.

Related Question