I have the same problem after a short cut the SSD wont boot, The SSD lights up as an external HD but wont boot in other macs or recovery mode. I think it also has something to do with Mojavo and the security mode. I read somewhere that sometimes Mojavo /macos can activate the disk as unreadable as a security measure when it is not proper ejected without a password etc. Dont have enough inside knowledge in this matter but since my SSD disk wont show up properly after one restart it seems more and more unlikely that it is a cable issue but more and more a Mojavo/Macos security precaution.