you cannot, sorry. iCloud kind of acts like a hard drive in the cloud, and if you want more storage you can use a service like dropbox to store files and then access them on your iPad Mini through the dropbox app: http://db.tt/ZEDL1EEE
Theoretically, you can, but only to transfer photos and videos that follow a very strict naming convention, and even with that, i've literally had a 50/50 success rate in getting USB flash drives to work (most want to pull more power from the iPad than the iPad's programming allows)
There is no support at all for the transfer of data, documents or music or the such from the USB to the iPad, and zero support to transfer anything off the iPad.
The camera connection kit can only be used to import photo/video files from a camera, SD card or USB flash drive. You can't import other types of files. You can't export any type of files using the camera connection kit.
There are some wireless external hard drives that can be used with the iPad.
The Kingston Wi-Drive, which costs $50 for the 16 Gigabyte, and then $30 more for every 16 gigs more. It works by you turning it on and then accessing the files on it from an app that you download on your iDevice. You can access music, movies, and other stuff. No connections or anything, it works like a WiFi connection, you connect to it from the setting on the iPad under wireless networks.
Then there is the Seagate GoFlex, which some would recommend over the Wi-Drive. But this one costs $199 and had 500 Gigabytes of storage. It works the sameway as the Kingston: no wires, runs over its wireless connection. You can actually fit up to 300 HD movies on it.
Expand your iPad's storage capacity with HyperDrive
On the road with a camera, an iPad, and a Hyperdrive
My daughters have just given me an iPad Mini for Christmas, thinking it would be useful as a backup device for my photgraphs while travelling. Sadly, I can't see - even with the information above - that it's going to be any use at all, and I haven't the heart to tell them.
Here's my situation: When I travel, I average about 200 photos/day. I shoot RAW, so each image is ~25MB. So that's about 5GB/day. In the past I've backed the days images onto (1) Wintel 11" laptop, then from there to (2) portable USB drive, and (3) a second portable USB drive. When I fly, I pack the laptop into one suitcase, I pack the first USB drive into a second suitcase, and I pack the second USB drive into my carry-on gear. Seems overkill maybe, but I'm not going to rely on a single point of failure for all those precious images.
In a 4 week period I accumulate about 150GB of images, and each is backed up in triplicate. The question is: can I achieve this level of backup with an iPad Mini? Everything I've read tells me "No".