It’s possibly down to the hard disk timeshift buffer file being corrupted due to faulty sectors on the hard disk.
There are a number possible solutions.
I install the custom firmware and rename the 0.ts buffer file which will force the box to create a new one on a different area of the hard disk.
2 Remove the drive and connect to a PC either booted into Linux or with a EXT3 driver (EXT2FS) and do the same.
3 Fit a new hard drive and use the old one in a sata usb cradle to view/copy the existing recordings.
4 Use the file system check and repair capability the custom firmware provides.