How can I connect an external hard drive to my ipad

You don't and you can't. You need to save your iPad content to your computer iTunes and delete the stuff you don't want to make space.

It's easier to move some of your data to the cloud.

Dropbox

http://itunes.apple.com/sg/app/dropbox/id327630330?mt=8&ls=1

Sugarsync

http://itunes.apple.com/sg/app/sugarsync/id288491637?mt=8&ls=1

Box.net

http://itunes.apple.com/sg/app/box-for-iphone-and-ipad/id290853822?mt=8&ls=1

People usually use cloud to tranfer files and such. You can't add extra storage.

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 wireless connection. You can actually fit up to 300 HD movies on it.

Another option:

Expand your iPad's storage capacity with HyperDrive

http://www.macworld.com/article/1153935/hyperdrive.html

On the road with a camera, an iPad, and a Hyperdrive

http://www.macworld.com/article/1160231/ipadhyperdrive.html

 Cheers, Tom 😉