external usb drive and battery drain

What would be the effect on battery run time of running a 4200rpm 2.5" notebook drive in an external enclosure such as this . Will it significantly drain the battery? Thanks.

I have a 60gb drive in an enclosure similar to that, and haven't noticed much in the way of decreased battery life.

The one thing I DID notice however: Make sure you get an enclosure that plugs into two USB ports (For the extra power), or else the computer will disable the port on spinup due to the high current draw.