Harddrive won't sleep.

Hi All,
I've owned my eMac for about 3 years and this is the first problem that I've needed help with. Shows how simple they are

I've got a Maxtor 1 touch 2, 200GB external drive and it used to behave itself with OSX 10.3.9 but now I'm on 10.5.2 and using the Maxtor for Time Machine, it won't shut down when I put the Mac to sleep. When I shut down the Mac, the Maxtor will shut down sometimes but not every time.
Checking or unchecking the "shut down drives when possible" box makes no difference.

I suppose I lost something when I changed the OS as I did an 'erase and install'.

Anybody got any ideas?
TIA.

P.S. Maxtor does shut down after 1 hour, by which time the whirring has driven me up the wall!!!