There is a small, removable section of the case on the back that covers the standard USB 2.0 port on the back. A 3G device is designed to socket in and replace the removed section of cover. (See pic above for reference)
I used an exacto knife and cut away a small section of the removable cover piece to let me seat a usb flash drive here and put the cover back on. I also plugged in an external hard drive and the unit powered the drive with no issues. I also tried an external USB CD drive and it worked fine also.
Note: The external drives plug in to the port but sit alongside the tablet when in use
There is also a micro usb port on the unit that acts as interface and charging point. The same charger for my blackberry charges the tablet up with no issues. The micro usb can also be used to transfer data to/from a PC and also be used as a host port (with an adapter cable) and only on extremely low powered devices or ones with an external power source of their own.
If you want, you can also get one of these with its own internal hard drive, but the unit is a little more expensive and slightly more heavy than the flash based ones.