Maybe you can find out what the problem is using the Reliability and Performance Monitor in the Disk section. To start this tool click Run in the Start menu, enter perfmon.msc and click OK. Now expand the Disk section and look which processes do the most I/O with the disk you want to shutdown.
I am trying to get HDD power management to work – i.e. I want HDDs to spin down when not in use. I set HDDs to turn off after 15 minutes of inactivity, but they always remain on.
I checked the usual things. I don’t have indexing or the defragmenter enabled, and I don’t have any anti-virus software. In fact, it’s a pretty clean install with just drivers and Firefox on it. I did enable SuperFetch but Vista spins down drives with it enabled so I don’t think it’s that.