Drive missing in Disk utility I have a 2012 Mac Mini with an OWC disk doubler installed with the OS on an SSD and Time Machine on the original HDD. I was attempting to install a fresh copy of El Capitan from an installer image on the desktop to an external drive. When the install failed the first internal drive (the SSD with the installed MacOS) has disappeared from Disk Utility and the diskutil list in the Terminal.

Last login: Sun Feb 23 17:19:14 on ttys000
El-Capitan-on-HDMI-Mini:~ dougandlynn$ diskutil list
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                  Apple_HFS TM Backup               999.9 GB   disk0s2
/dev/disk1 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *4.0 TB     disk1
   1:                        EFI EFI                     314.6 MB   disk1s1
   2:                  Apple_HFS All Media               4.0 TB     disk1s2
/dev/disk2 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *480.1 GB   disk2
   1:                        EFI EFI                     209.7 MB   disk2s1
   2:                  Apple_HFS MacMini QC HD           479.1 GB   disk2s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk2s3
/dev/disk3 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk3
   1:                        EFI EFI                     209.7 MB   disk3s1
   2:                  Apple_HFS WD 1TB                  999.8 GB   disk3s2
/dev/disk4 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *4.0 TB     disk4
   1:                        EFI EFI                     314.6 MB   disk4s1
   2:                  Apple_HFS BU All Media            4.0 TB     disk4s2


This happened once before and I got the internal SSD back by erasing the drive which I had installed the new OS on. I tried that process and this time I still am missing the drive. I'm positive the SSD has not failed it's just missing. Any idea on how to make it reappear?
Either the SSD, or its connecting cable, or the underlying Mac Mini (either hardware or software) has a problem. 

Isolate the issue by taking the SSD out of the Mini, and connecting it to another Mac. If the drive mounts, that suggests the connecting cable or the underlying Mac Mini is the problem. Keep swapping parts until you find the bad link in the chain. 

Cables are fragile. If there are now two drives in the Mini and the other drive works, try swapping the two drives' cables to see if the SSD's cable is the problem.