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.
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Thank you for your input:
The Kingston is too small, and actually expensive per GB.
I still don't know if the Seagate can communicate Page and Number files with the iPad. ??? I have a bunch of Excel and Word files that Apple's Page and Number apps will read and be able to edit, so these are the kinds of files I'm talking about wanting to save on an external drive and available to the iPad.
I don't THINK the seagate will do that. I'd ask them for sure or look on their site, but my impression of that drive is that it's mean to be a media streaming device....store your movies on it, access them via the wifi on the drive with the iPad. I'm not under the impression that you can actually use the iPad to write files to the drive.
Since the iPad doesn't have a file manager, there's no support for it to manage and move files. (this is what makes it more virus resistant than other machines).
Those two drives that macman pointed out are the only two that I've seen that support reading and writing with the ipad. And that support is largely because the app that goes with them supports the moving of files.
There are a few other options, such as something like Dropbox where you store documents out on the cloud. Or I have documents to go which has a separate desktop program to sync documents outside of iTUnes, but in the first instance you need internet connectivity, in the second, both the computer and ipad have to be on the same wifi network.