External Usb Hd Auto Spindown?

Have had a Western Digital 500GB USB external HD now for a couple of months.

Just plugged it in and it was fine. Even auto spindown was working. That is it, goes into idle mode if you haven't accessed. (Not sure of the time parameters here) But I would estimate 2-3 mins and it stops. If you access it again, it takes about 3 secs to come back to life.

For some reason this feature seems to have stopped working. It would seem to be good to have it working. But I really don't know where to begin.

Any guru's out there?

Thanks