SSD mounts and unmounts itself

I don’t think it is the SSD itself, I booted into drive selection and the MBP SSD appeared and stayed there for over an hour. I think if it was something on the SSD, it would behave in a more random way when it unmounts itself. But in my case it is always a few seconds, never faster, never slower. It would also probably mount, stay mounted and give an error when trying to access it (or some specific file).

Perhaps is something with the files on the drive, something that tells the OS to unmount it when it reads it, or macOS doesn’t like two APFS boot drives connected at the same time. Or maybe some weird bug in the second macOS.

Second Mac has no FW ports and I don’t have Thunderbolt cables on hand.

If by single out the problem you mean check if the SATA connection is working properly, I can tell you yes. I installed a HDD in place of my MBP SSD and it showed up on Finder as it should. I also swapped both drives, installing the MBP SSD in place of the second Mac’s drive and it booted from the second Mac’s drive just fine. I don’t believe that the issue is hardware based.

Well, that kinda defeats the purpose of using the second machine to clone the MBP SSD... But the second machine works normally without the MBP SSD. In fact, it works normally even with the SSD installed, apart from the fact that it unmounts by itself.

Installing other drives instead of the MBP SSD also works like it should, which leads me to believe that my issue isn't hardware based, and is specific to that SSD.

I know ;)

I'm just out of ideas on how to solve this. Honestly, if I could clone this drive and copy some files from it, I would format it to see if that solves the issue. I'd restore this data from my Time Machine backup, but unfortunately I can't do it due to the drive in the other Mac being a LOT smaller and because I want an exact copy of the Mac SSD in another drive.

It isn't failing, First Aid said so, and df -h didn't list unmounted drives.

I tried diskutil list, that showed me all disks and partitions. Trying to repair it yields the same result from First Aid:

The volume /dev/disk1s2 appears to be OK

What I mean is that both drives w/ 10.13 show up when holding Option at startup. I think that if the MBP SSD had a problem, it wouldn't show there either.

It really seems like macOS simply ejects the SSD seconds after I mount it.

Yeah, that won't work. Both require a mounted drive to do the process. Mine won't stay mounted long enough to do it.

I also don't think that it would help either, my issue isn't about mounting, but about staying mounted.

Internal SATA connection. Swapping my two drives positions doesn’t solve the issue.