OS 10.5 - External hard drive wakes computer from sleep

I'm having this issue ever since I upgraded to Leopard. My computer always wakes from sleep no matter if I manually put it to sleep or it goes to sleep via the power saving settings I have.

Looking at my console logs, I can see that something plugged into the USB is causing it to wake:

13/02/08 11:00:07 PM kernel System Sleep
13/02/08 11:03:53 PM kernel System Wake
13/02/08 11:03:53 PM kernel USB caused wake event (EHCI)
13/02/08 11:03:55 PM kernel AppleYukon2 - en0 link active, 100-Mbit, full duplex, flow control disabled port 0
13/02/08 11:04:04 PM mDNSResponder[20] Note: Frequent transitions for interface en0 (192.168.0.4); network traffic reduction measures in effect
13/02/08 11:04:33 PM ScreenSaverEngine[493] Screen saver is running in blackout mode.


The only 2 things that are plugged into the USB ports are the keyboard/mouse and an external hard drive for Time Machine. I unplugged the hard drive, put the computer to sleep and for the first time since Leopard was installed, the computer stayed sleeping all night.

Is there something that can be done to keep my external hard drive on and plugged in while the computer sleeps or will I need to unmount it and turn it off every time I want it to sleep?

Is this a hardware issue or software issue?