Q: Has anyone found an external hard drive that will work with the camera USB adapter on the iPad?

There aren't any. There's no support for file transfer via USB barring importing photos and videos from a camera (or flash drive or SD card that you fool the iPad into thinking is a camera)

Look into some of the wifi enabled portable drives. The Seagate GoFlex was one early model but I'm sure there are more now. These will be drives that have their own battery operated wifi transmitter that you connect your iPad to, then use an app to transfer files (since the iPad doesn't have a file manager)

You could be brave and see if the iFlash drive would work.  You will need to put a powered usb hub between.

Here are some more:

Airstash
"Easily access, transfer, and stream your files from AirStash® – the pocketable wireless flash drive for iPad, iPhone, and iPod touch! This app requires an AirStash device in order to work.  View, import & export photos, wirelessly stream videos, music & podcasts, and view documents stored on your AirStash. AirStash+ is an enhanced alternative to the built-in web interface adding features only possible with a native app."
http://www.airstash.com/

Seagate Wireless portable hard drive
"The new 500GB Seagate Wireless portable hard drive is the company's latest HD designed specifically for use with tablets and smartphones like Apple's iPad and iPhone."
http://www.macrumors.com/2015/01/04/seagate-ces-2015-new-products/

FlashAir™ II Wireless SD Card series

--Share photos & videos wirelessly

--Capacity up to 32GB

--Supports simultaneous access from up to seven devices

http://www.toshiba.com/us/wifi-memory-cards

iFlash Drive
Here is a nice adapter.  You can move data in both directions!
http://www.ebay.com/itm/i-Flash-Drive-Dual-Ideal-for-Adding-Extra-Storage-Device s-for-iPhone-iPad-iPod-/321550878374?pt=US_Memory_Card_Readers_Adapters&hash=it e m4added06a6

I have an iFlash drive and can confirm that if you download and install the iFlash app from the Apple App store you can transfer/export/import files to and from iPhone (5), iPad, etc. The app is or used to be free. An alternative is to install Dropbox on your iPhone/iPad etc allows you to upload/download files online and then access them offline.