There are external storage devices for the ipad. These change all the time.
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/
" SanDisk Connect Wireless Stick is the flash drive reinvented to work with both your computer and your mobile devices."https://www.sandisk.com/home/mobile-device-storage/connect-wireless-stick
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/