Is my SATA cable damaged? 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.
I replaced the cable, and everything is now back to normal. So, as I suspected, the cable was the culprit.

Instructions on how to replace the SATA cable is easily found online 

(search for things like replace hard drive cable macbook).

The only tricky part was to connect the sleep LED to the cable, but if you just check how the old cable is connected you'll be fine.