Recently my 4TB external Seagate drive started having problems. It would disappear from Windows Explorer. After restarting, it'd work for a while. Eventually the drive slowed down so much that it basically hung the computer.
I've started using ddrescue to clone the drive to another Seagate 4TB drive of the same model but different revision. So far, after a day of running ddrescue, it's still on pass 1, 150,369 MB rescued, 3.75% rescued, 224 read errors, 0 bad-sector, 0 bad areas, remaining time 39d 21m. The only kind of error, and there's a lot of them, is "critical medium error, dev sdc, sector XXXXXXXXX". The sector number goes up every 500,000 - 640,000 or so. It hasn't reported any other kinds of error. Average rate is 840 kB/s. So far the errors seem to be spread out over the disk and not concentrating in clusters.
The disk is not very hot, and doesn't make much sound. The destination disk is in the computer connected with SATA cable.
Since the failing disk doesn't have any critical data on it, I'm willing to let ddrescue complete. What I'd like to know is what kind of disk failure would this indicate, and what exactly is critical medium error, sector XXX?