Macbook Pro Mid 2012, original SSD.
TLDR; Why could I clone my unmountable drive from an external enclosure but not internally?
My SSD broke down a few days ago. It was detected, but I was unable to mount it, and I got I/O errors when trying to clone (restore in Disk Utility) the drive (no logs saved). After Disk Utility and DiskWarrior failed to resolve the issue I put the internal SSD into an external enclosure. From the enclosure I was suddenly able to clone the drive. My question is, what possible explanations for this are there? My guess is that the SATA connector is damaged, but I hope I'm wrong.
Update: A fresh install (made from the external enclosure) still has the same mounting issues when booted internally.
Update 2: After a few more hours of research I'm more or less convinced that the cable is toast. Apparently it's a common occurrence with certain Macbook models.