Are you sure you tried to mount the correct drive? Internal drives on your computer would have carried a designation of "hda1", "hda2" for a primary drive, or for a secondary drive "hdb1" etc. An external USB drive would have been "sda1", "sda2", or "sdb1", etc. Once you have mounted the correct drives you should see a list of your files on the computers hard drive on one window and your USB on the other - it should be a simple matter of drag and drop to copy your files.