The easy fix would be to watch any important recordings then format the disk. The complicated fix would be to take the drive out of the Humax, attach it to a PC running Linux (eg from a live boot CD) and run e2fsck.
Out of interest, would backing up recordings work as an alternative to watching them?
You are right of course. I guess you could FTP them from the box and then put them back after the format or the slower route of copying to an attached USB storage device. The encryption keys are not linked to the hard drive.