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