Time Machine drive runs when not backing up..

I've noticed this before but, today, I had to do a full restore from my Time Machine drive. It is a WD 500Gb using FireWire. After the restore I let things run long enough for a backup to complete.

I later noticed that the drive head was working consistantly and turned Time Machine off. The drive has continued some operation for quite some time though Time Machine remains grayed out.

Can someone offer an explanation of what is happening in the background and whether it is OK? I used to gete paranoid when this happened on my Windows machines.

It sounds like the WD drive cache was still working and catching up writing data. Firewire transfer is fast - especially FW 800 but when you're moving that much data it's going to take some time to complete. Just don't unplug or try to dismount it while it's doing that as it could result in drive errors and possibly do sector damage.

Regards.