a couple of days ago had this issue:
Decided to copy everything from the ssd (and the internal hd), get a new one (and a new hd too).
Earlier today I realized there was a folder in the hd I still hadn't copied or backupped earlier.
Plugged a portable Seagate, and dragged the folder.
Copying started, until it seemed stuck while copying a photo, estimated time left changed from a couple of minutes to 5 hours, Openbox windows turned transparent and copying stopped (in the meantime Firefox, terminal... opened and worked without problems).
Killed the OpenBox process, restarted and everything pretty fine... except that now the hd (NTFS) dosen't mount anymore, if I try to mount it I get an error message that blames an "Unknown Error".
The hd doesn't mount on another pc with Manjaro Mint, neither on a laptop with Sabayon Linux.
It does mount if I boot the same laptop in Windows 10...
However not that practical when I'll have to copy back all that stuff on the new ssd and hd...