As I stated before, uPNP is turned off. There are no backup programs installed on any of my computers, and I have no UPS, just a power bar which seems to be functioning normally (it's a Belkin unit, if that helps).
MountainMan seems to have a similar issue, though my unit seems to go into standby for at least an hour usually before resuming (I haven't timed it yet to see if it's always the same length of time). Like I say, there seems to be no outside influence here, so it must be something in the device itself which makes it resume from standby. When it does resume, it only makes a very short seek, usually about 1 second of noticeable hard drive activity, then waits the set 30 minutes before going back into standby. Very strange...
edit: oh, and no, I'm not using the print server, my printer is hooked up to another computer.