This can also be caused by a USB controller or HD controller failure which is not allowing WIN to read the drive properly. First, try using a different USB port on the PC. If still reading similar, than most likely the issue is within the drive itself.
I'm not a big fan of incremental backups as you need the original backup, and all incremental backups to successfully restore. Any one incremental backup which gets corrupted, could potentially cause the restore to fail.
Differential backups only require the original backup, and the latest differential backup to restore. Only involves 2 files, instead of how many incremental backups were made?