HDD slow contents display

I have 2 internal drives in my G5, and one FW800 external. In the Finder, sometimes I get the spinning ball for about 5 seconds when clicking on my second internal drive (Work Disk) before it will display its contents. This never happens with the other two drives. They are all 7200 rpm drives with similar specs. I think it is the OS. Any ideas why this happens?

Is it the disk spinning up from not being used?

That's very likely, as it obviously isn't accessed as much as the system disk. Is there a way to be sure it's always ready? I ask only because this is the drive I record multi-track audio to, and it shouldn't be sleeping.