What Shadorne said. If you do that it may repair permissions on the FreeAgent. That's what your problem sounds like off the top. Permissions allow the various users of the computer to read only, write only, read & write, etc.
Transfer of Library to a back-up External HD
My iTunes library (about 500 gigs) is loaded on a Raid-1 Cavalry external HD. In an attempt to get additional back-up I purchased a new FreeAgent Desk 1 TB external HD. The Mac recognizes both HDs, however when I attempt to click and drag from the Cavalry to the FreeAgent the response on the monitor is, in so many words, the transfer can not be completed because the FreeAgent cannot be modified. This was supposed to be super simple according to the Geek at Best Buy. Am I doing something wrong here? The Cavalry is connected to the Mac via Firewire, the FreeAgent is connected via USB.