I have a 2013 Mac Pro running 10.13.4. The internal ssd (1TB) suddenly stopped booting. Luckily I have a clone which is only 2 days old. The internal drive will not stay mounted. If I do a cold start I can boot off the ssd but after using it a few minutes the drive unmounts. I can boot from the clone. The ssd shows up but after several minutes it unmounts - I get the message saying it was not ejected properly. I tried disk utility after booting from the clone. It freezes up while doing first aid. I try to erase it but I either get an error saying it cannot unmount or else nothing happens and I finally must force quit disk utility. I can't try disk warrior as it is an APFS disk. I can't try clone back from clone to ssd as it won't stay mounted. I could buy another ssd but a 1 TB costs almost a $1000 and I am not sure sure if that is the problem. Any suggestions?