[CentOS] Centos4: USB disk standby?

Hi,

I've decided to use an external USB disk as backup with rsync.
This works fine.

When the external disk is mounted, it does not go into 
standby/sleep/whatever, though I'm using it only for 30minutes at night.

Is there a way to do this with C4, 2.6.9-78.0.8.ELsmp?
E.g. unmount/some special command/upgrade to 5.2?

Thx
Raier