ubuntu-server 7.10, what might be preventing my USB drive from sleeping?

[Chris G]

I have a new ubuntu server 7.10 installation used as a NAS system in
my garage.  It has a 500Gb external USB drive connected to it.

Something is preventing the external USB drive from going into sleep
mode, i.e. something is accessing the USB drive at intervals such that
it thinks it shouldn't power down.  Previously when I used the network
drive via its ethernet interface (not from the ubuntu server system)
it did go into sleep mode.

The ubuntu server installation was done without the USB drive even
connected so there's nothing apart from backups on the external drive.

Can anyone suggest what might be keeping the USB drive awake and/or
ways of diagnosing what's keeping it awake?

-- 
Chris Green