To add to the above, there are two faults: both seem to be related to retunes. One fault (discussed in the link posted in BH's message) causes a flag to be set incorrectly in the HMT file. This affects both SD and HD channels and gives a 'channel scrambled' message on playback. If you have custom firmware this is easily fixed by running the 'fixencflags' diagnostic.
The other fault (mentioned in the same thread as above; useful link
here) affects the newer HD channels, e.g. BBC Three HD and BBC Four HD, with recordings appearing to be zero length and not playing. This fault is caused by corruption of the NTS file and can be fixed automatically on the HDR-FOX with custom firmware. On the HD-FOX it is not so easy. I have not tried this entire procedure myself, but if you have custom firmware, install the BootHDR package and boot into HDR mode. Then if you copy an affected recording from the simulated 'HDR' folder to a different destination on 'drive1' (the contents of the 'Video' folder of 'drive1' will also appear in the 'My Video' folder of 'HDR') using the remote control of the HD-FOX, the programme will be decrypted on copy. If you then delete the sidecar files (NTS, HTM and THM) from the copy it should now play, but with limited transport controls.
I have read on this forum that if you manually record from the affected HD channels then subsequent timer recordings work OK until you do another retune, but I can't find the post at the moment.