CCC supports these configurations:
* Local (i.e., not over a network connection), HFS+ formatted partition or hard drive.
* Mounted disk image. Cloning to a disk image will (obviously), not yield a bootable volume unless you use CCC to restore the image to a physical partition or disk.
* Firewire disks including iPods
* CCC will not backup directly to CDs or DVD-R discs, though you can backup to an appropriately sized disk image, then burn the image to disc with Toast or Disk Copy.
* Any machine that supports Mac OS X (officially)..
Notice that it won't clone to an external drive if it isn't FireWire. USB & USB 2.0 aren't supported.
The reasoning for this is kind of basic. Macs (PowerPC based, at least) can't boot from USB, thus you can't clone a boot volume to a USB drive.
If all you need is access to your old files, then just copy the folders that contain your data. It'll save space on your external drive that would otherwise be wasted on cache files, etc.