I have my music collection ripped to MP3 on an 80gb external drive, but it is nearly full. I am considering buying a 400gb external drive and transferring the music over from one drive to another. Can I connect both drives to the pc at the same time and do this relatively easily?
Yes you can. They will be assigned different letters - drive G and drive H, for example, and you just treat them exactly the same as your other drives, transferring files from one to the other.
You can always unplug your USB printer or USB webcam or similar while transferring the files and plug them in again afterwards. This eliminates the need for a hub.