Now there's an interesting thing - BOAC's reply is 12 minutes before PP's question!!!
Telepathy rules in advance!
Back to the question (or is it the answer...)
Pop, it can often be that one drive is taking too long to spin up - not necessarily a problem with the drive - it's as likely to be a problem with the PSU, especially if you have several physical disks and CD/DVD drives!
There's a time limit for the BIOS to identify drives at boot - if one of them hasn't reported in, it will then not be visible to the BIOS and therefore not to the OS.
It is possible on some (newer) mobos to specify a longer drive seek time at boot (up to 35 secs on mine). I have pared it back to 20 secs - 15 is too short, and it doesn't find the drive.
SD