Ok Stuart, let's try and get to the bottom of this annoying problem. As you may have gathered, I also get this problem occasionally, and as far as I can recall it started after I had tried to install XP SP3. The network drives concerned on my system are two memory card slots built into my HP Officejet printer. My system is a Packard Bell Imedia 1208 (or something like that). In my case, it doesn't appear to cause a major problem because if I look at My Computer after system startup the memory card on HP shows as available / active.
As I said, I get this problem from time to time, with no discernible pattern, hence my earlier suggestion that it was a YYGT.
TTFN
Mick