A pure guess is that the hard disk time shift buffer file area of the hard disk has got corrupted.
A format of the hard disk may fix it, but will lose all your recordings.
The best solution is to install the Custom Firmware from AVForums, this will give you the capability to rename the time shift buffer file 0.ts to something like 0.bak. This will cause the box to create a new 0.TS on another area of the hard disk.
Probably time to consider replacing the Hard Disk Drive,