Guest Charles, I did indeed quote incorrectly from the destruction manual: It should have been a 500Gb HDD.
I can only plead that the bright revelation of the probable reason temporarily blinded my cortex, causing a typographical error.
And the fact that I was translating chinglish.

The problem arose because, although the STB unit has recorded faultlessly for approaching 6 months on the 8Gb flashdrive, TV channel programming changes have necessitated a larger storage unit, to take 3-4hrs of video. Assuming that, if it worked on 8Gb, surely it would work on 16Gb....
No it didn't.

Sorry about the delay in replying, I've been out all day. 7.30pm here now.

Chris.