As for my old HDD, I left it connected inside my Win 10 PC for a while and suddenly a File Explorer window comes up showing the drive being read. I did not run any chkdsk function yet, so it is strange to see the HDD suddenly spring back to life.
I pulled out the 860 out of my laptop and put the HDD in and lo, it is booting normally. Everything was normal, just as the 860 before that. But after a restart and re-connection of an external drive and an Ethernet cable did the old problems reoccur. Explorer.exe crashing loop and things being slow in general.
It's okay. I've been wanting a recovery for the drive and data, which has been successful, and the 860 is standby in case the HDD really kicks the bucket. So overall there is nothing to worry about. I am uninstalling AVG, DropBox and other depreciated unnecessary crap so I can squeeze the most out of the HDD.
My most sincere thanks to you all. I'd kiss you all on the head if I could, but I can't. So please enjoy this instead:
https://www.youtube.com/watch?v=8CCf7gvmDEU 