Thanks for the responses - I agree... it definitely seems like a spool-up delay. The setting is definitely disabled though.
In case it's a corrupt setting, I enabled power management, set it for 10 minutes, set it for 5 minutes and then disabled it again. I'll leave it overnight and will check it again tomorrow.
I'd love to physically check the drives but the NAS is located in an area which makes it impractical to do so. If I still have this issue, I'll remove the NAS from its location and will be able to physically check it at that time.
Jack: When you say you have the same issue with your 320s and 325s, do you have the power management enabled or disabled? I'd very much like to get confirmation as to whether this is a firmware issue.