If you're running any software (eg, SpeedFan, AIDA) that monitors the disk temperature (via SMART), it will spin up the disks even when they are idle.
I tried several times to get XP to spin down my idle disks, and eventually gave up. In my situation, it was best to let them spin all the time.
Thank you, I'm not running Speedfan or AIDA, but maybe Win7 keeps checking on the disks' status via SMART ? If so, I'd like to know how to stop it. I certainly don't think there's anything wrong with the HDs, the Z7K500 is the kickass-est 2.5" out there.