I am trying to create a low power NAS based on a Via C7 board. I had a spare XP licence so decided to use that for simplicity and compatibility with other Windows machines.

The machine will enter S3 sleep state fine when not in use, but when it wakes up it spins up all the HDDs again. This seems a bit silly, since usually only one HDD is being accessed.

Is there a way I can prevent it spinning the drives up again?

The alternative is not to use sleep modes at all and keep the system running 24/7, but of course that uses quite a bit more power than S3.