Here's a flash drive that works both ways with iOS - Mac/PC and has an app that it is used with it for iOS.
The app is called iFlashdrive.
It seems somewhat expensive but it is definately worth it in the long run.
$99.99 for an 8 gig
$149.99 for a 16 gig
$199.99 for th 32 gig
They also have an FTP drive that you can plug in any USB storage device and connect via wifi with the device, but you do not need to be connected to conventional wifi or 3G, etc. To me this sounds like a better option. It retails for $99.99 at http://www.hypershop.com/CloudFTP-by-HyperDrive-s/218.htm.
Good luck, and I hope this helps.