SSD mounts and unmounts itself

I have a SSD that was my boot drive in my now-dead MacBook Pro, and I need to recover some data from it and clone it.

I installed it inside another Mac, however, when I boot into this Mac, the MBP SSD unmounts itself after a few seconds. Mounting it through Disk Utility ends up in the same way: it mounts and after a few seconds, it unmounts by itself. Doing it through Terminal says the disk was mounted successfully, but the same thing happens, the drive unmounts itself.

First Aid the drive is fine, and swapping the MPB SSD with another drive still results in the same issue.

Does that happen because the Mac sees two bootable drives or something like it? Both machines are/were on High Sierra w/ APFS.