Aha. 4gb maximum file size on a FAT32 flashdrive. That will be it. Thank you, Charles.
I still do not understand why it has a tea-break when reaching its maximum file size, though. If it had just turned off-then-on-again, you'd hardly notice, but we lost the last bit of 'CSI' and the first bit of 'House.' Ruined both programmes. Two weeks in a row.

Having now (in desperation) read the destruction manuals, The STBox requires a FAT32 system but suggests a HDD, 500mb with 4 partitions.

My personal chancellor of the exchequer seems fairly OK with the purchase of a HDD, having had it explained to her. "But not off the web!"
Spoilsport.
Chris.