I have a lot of photos on my hard drive that I'd like to keep. I also have office documents I'd like to save, too. I was wondering what I can purchase to download my files and save them so that I have another copy. Preferably something I can plug into my usb port. I've heard of zip drives before but not sure what they are or how to use them. I don't want something I have to plug into the wall, though. I want something that will be seperate from my computer. Any ideas?