Android tablets will not accomodate an optical drive, and most of them will already achieve a full day of battery life. However, an extended battery pack (or possibly a spot to mount an SSD for one of the ones with a USB host port) is never a bad thing.

If somebody were buying a new Android tablet, however, and storage space were critical to them, it would impress me if they didn't know about this tablet that has a 250gb spinning hard disk in it.

Accessories like this have been being built pretty specifically for the iPhone/iPad/iPod-Touch for a while now, but this version seems to just be a box with USB ports on it. Users of iOS devices must be completely uninterested in adding functionality or battery life to their devices.

Of course, those still leave it upon you to have a wallet that's not built into a gadget.