You cannot connect your HDD to the iPad like you do with a laptop.
Have a look at the Seagate GoFlex Satellite
There are some wireless external hard drives that can be used with the iPad.
The Kingston Wi-Drive, which costs $50 for the 16 Gigabyte, and then $30 more for every 16 gigs more. It works by you turning it on and then accessing the files on it from an app that you download on your iDevice. You can access music, movies, and other stuff. No connections or anything, it works like a WiFi connection, you connect to it from the setting on the iPad under wireless networks.
Then there is the Seagate GoFlex, which some would recommend over the Wi-Drive. But this one costs $199 and had 500 Gigabytes of storage. It works the sameway as the Kingston: no wires, runs over its onw wireless connection. You can actually fit 300 HD movies on it.
Expand your iPad's storage capacity with HyperDrive
On the road with a camera, an iPad, and a Hyperdrive