I don’t know of any external hard drives that you can attach to an ipad. You’ll have to use a regular computer as a data shuttle. Other than that there is not really anything to know. They’ll work like a big ass thumb drive.

Pretty much. A computer will treat an external like a thumb drive, and an iDevice won’t handle external drives at all.

However, an iDevice can only really store media files, and not in a way that can really easily be used to transfer them from computer to computer; it will only ever work with one registered iTunes account. The manner in which it does so means that either the files you want to save are already in the iTunes library of your MacBook, making an external drive irrelevant, or things will get complicated as you try to do stuff Apple doesn’t like letting people do. You should be able to sync your iPad to grab whatever media is on there, but unless iTunes has gotten more forgiving, that’s all you can really do.