iMac 27 How to restore a SSD made invisible after formatting a fusion drive from APFS to HFS+
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