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I bought a used Imac 27 with APFS fusion drive mOdel Identifier iMac15,1 before discovering the previous owner had replaced the original 512 Gb SSD by third party one with an adapter. After formatting the fusion drive back to HFS+ with disk utility, and installing OSX 10.12.6 (Sierra) with my previous data from my older computer, the SSD became invisible both in disk utility and diskutil (see the first lines of the "disk utility list" command. I have made additional investigations, and determined that booting from a High sierra or above OS the SSD is again visible, and I was able to partition it, make an HFS+ Fusion drive with the HDD or a partition of the HDD, etc all operations appearing normal. However when booting again from Sierra, neither the SDD nor the Fusion drive are visible in diskutil or disk utility. I want to stick to Sierra for a number orf reasons including compatibilty with other older machines. How could I restore the SSD to become visible and useful again ? Just below are the first lines of the result of the command system_profiler SPSerailATADataType (after upgrading again to OS 10.15.5 system_profiler SPSerialATADataType SATA/SATA Express:

Intel 8 Series Chipset:

  Vendor: Intel
  Product: 8 Series Chipset
  Link Speed: 6 Gigabit
  Negotiated Link Speed: 6 Gigabit
  Physical Interconnect: SATA
  Description: AHCI Version 1.30 Supported

    ST4000DM006-2G5107:

      Capacity: 4 TB (4 000 787 030 016 bytes)
      Model: ST4000DM006-2G5107                      
      Revision: DN04    
      Serial Number: ZC19X1NJ
      Native Command Queuing: Yes
      Queue Depth: 32
      Removable Media: No
      Detachable Drive: No
      BSD Name: disk1
      Rotational Rate: 7200
      Medium Type: Rotational
      Partition Map Type: GPT (GUID Partition Table)
      S.M.A.R.T. status: Verified
      Volumes:
        EFI:
          Capacity: 209,7 MB (209 715 200 bytes)
          File System: MS-DOS FAT32
          BSD Name: disk1s1
          Content: EFI
          Volume UUID: 0E239BC6-F960-3107-89CF-1C97F78BB46B
        disk1s2:
          Capacity: 4 TB (4 000 577 273 856 bytes)
          BSD Name: disk1s2


-bash-3.2# diskutil list
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *4.0 TB     disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                  Apple_HFS HD1-IMAC27              4.0 TB     disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3

/dev/disk1 (disk image):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        +2.1 GB     disk1
   1:                  Apple_HFS OS X Base System        2.0 GB     disk1s1

/dev/disk2 (disk image):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                            untitled               +5.2 MB     disk2

Below is the result from the two commands issued in OS High Sierra after successful creation of a Fusion drive formatted in HFS+ and with one partition of the HD. Yves:Imac-27- ~ yves$ system_profiler SPHardwareDataType Hardware:

Hardware Overview:

  Model Name: iMac
  Model Identifier: iMac15,1
  Processor Name: Intel Core i7
  Processor Speed: 4 GHz
  Number of Processors: 1
  Total Number of Cores: 4
  L2 Cache (per Core): 256 KB
  L3 Cache: 8 MB
  Memory: 32 GB
  Boot ROM Version: 235.0.0.0.0
  SMC Version (system): 2.23f11
  Serial Number (system): C02NLC33FY14
  Hardware UUID: 7791AAC4-ED40-53C8-AA90-41C48B10DD89

Imac-27-Yves:~ yves$ diskutil list /dev/disk0 (internal, physical): #: TYPE NAME SIZE IDENTIFIER 0: GUID_partition_scheme *4.0 TB disk0 1: EFI EFI 209.7 MB disk0s1 2: Apple_HFS HD2-27 2.0 TB disk0s2 3: Apple_Boot Recovery HD 650.0 MB disk0s3 4: Apple_CoreStorage Macintosh HD 2.0 TB disk0s4 5: Apple_Boot Recovery HD 650.0 MB disk0s5

/dev/disk1 (internal): #: TYPE NAME SIZE IDENTIFIER 0: GUID_partition_scheme 512.1 GB disk1 1: EFI EFI 209.7 MB disk1s1 2: Apple_CoreStorage Macintosh HD 511.8 GB disk1s2 3: Apple_Boot Boot OS X 134.2 MB disk1s3

/dev/disk2 (internal, virtual): #: TYPE NAME SIZE IDENTIFIER 0: Apple_HFS Macintosh HD +2.5 TB disk2 Logical Volume on disk1s2, disk0s4 582B2901-BB86-47DC-857E-14E742AED9F9 Unencrypted Fusion Drive

  • SSDs don't become invisible to the system in any way. All those "extra" drives are RAM disks needed for the install. That said...let's slow down some. I think your going into this without a clear picture of what you have in your possesion and how recovery works - that can cause major mistakes. First, let's figure out what you have and from the looks of everything, you have an iMac with a 4TB disk drive. – Allan Oct 23 at 22:43
  • We also need to know what iMac you specifically have. Please issue the command: system_profiler SPHardwareDataType | grep Identifier and post that info to the original question (not comments). Also with system_profiler SPSerialATADataType this will tell us what drives are installed. – Allan Oct 23 at 22:49
  • @allan After additional experiments, I have edited my question. I can confirm now that when booted from OS High Sierra and above, the SSD is visible, can be partitioned, formatted, can be used either alone or in combination in a fusion drive formatted in HFS+, (and probably other formatting options, not tried) but as soon as I boot from Sierra, the SSD is gone, and even the size of the Fusion drive previously created in High Sierra appears to shrink, excluding the additional capacity previously provided by the SSD. – Yves Oct 26 at 13:55
  • You need to provide the full output of the commands, not just the first few lines. Now, Sierra is not compatible with APFS so you won't be able to mount those volumes, but you will be able to see attached hardware. Hardware doesn't disappear under Sierra. Additionally, keeping "Sierra" because it's compatible with older machines" makes no sense. How are you defining this "incompatibility" with older machines? – Allan Oct 26 at 14:10
  • What specific Mac do you have? The 15,1 can be a late 2014 to mid-2015 5K iMac. Please, boot into recovery (Cmd-R) then run the commands I provided so we can get a clear picture of what's installed on your iMac. There's no need for all this "experimentation" at this time. – Allan Oct 26 at 14:12
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I have not exactly identified the source of the problem, but after upgrading to High Sierra (OSX 10.13.6), everything went back to normal behavior. I could see the SSD again, format it, and combine it with the HD into a new fusion drive formatted in HFS+, as was the original disk setup of the computer.

Imac-27-Yves:~ yves$ diskutil list

/dev/disk0 (internal): #: TYPE NAME SIZE IDENTIFIER 0: GUID_partition_scheme 512.1 GB disk0

1: EFI EFI 209.7 MB disk0s1

2: Apple_CoreStorage Macintosh HD 511.8 GB disk0s2

3: Apple_Boot Boot OS X 134.2 MB disk0s3

/dev/disk1 (internal, physical): #: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme *4.0 TB disk1

1: EFI EFI 209.7 MB disk1s1

2: Apple_HFS HD2-27 2.0 TB disk1s2

3: Apple_Boot Recovery HD 650.0 MB disk1s3

4: Apple_CoreStorage Macintosh HD 2.0 TB disk1s4

5: Apple_Boot Recovery HD 650.0 MB disk1s5

/dev/disk2 (internal, virtual): #: TYPE NAME SIZE IDENTIFIER

0: Apple_HFS Macintosh HD +2.5 TB disk2

                             Logical Volume on disk0s2, disk1s4

                             582B2901-BB86-47DC-857E-14E742AED9F9
                             Unencrypted Fusion Drive
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