I put together an article about using an external hard drive with an iPad. It does require jailbreak, but doesn't require special hard drives or anything like that.
Basically requires:
Link: http://www.pocketables.com/2013/02/how-to-use-an-external-hard-drive-on-an-ipad-video.html
Video:
Basically requires:
- Jailbroken iPad with iFile
- External hard drive formatted to FAT32 or other compatible file system format (not NTFS or exFAT)
- USB adapter (Camera Connection Kit for 30 pin, Lightning to USB Camera Adapter for lightning)
- Some way of powering the drive. Either a self-powered drive, powered USB hub, or a USB battery pack like in the video
Link: http://www.pocketables.com/2013/02/how-to-use-an-external-hard-drive-on-an-ipad-video.html
Video: