For "improved detachability", OS X will unmount any non-internal volumes that are attached to the system when you log out. So, for example, if you log out of your computer while a USB or Firewire hard drive enclosure is attached, you can detach those hard drive enclosures from the system without having to manually unmount them first. This is a good thing — it would be annoying if you had to log back in to your system just to eject a drive. The downside of this, though, is that if you have a scheduled backup task that runs when no user is logged in, the destination volume may be unavailable. For a local backup, CCC will attempt to manually mount the destination volume. When the destination of your backup task is a remote Macintosh, though, CCC will not be able to mount that volume prior to backing up.
If you anticipate backing up to a remote Macintosh that may be sitting at the loginwindow, you can change the behavior of OS X to not unmount detachable volumes. To change this behavior, run this command in the Terminal application on the remote machine:
sudo defaults write /Library/Preferences/SystemConfiguration/autodiskmount AutomountDisksWithoutUserLogin -bool YES
Note for Snow Leopard users: This workaround does not work on Snow Leopard 10.6.0 to 10.6.3. If you require this functionality, please apply the 10.6.4 update (or the latest available) for the best experience.