I just formatted a spare external to simulate how most of them come out of the box carrying a PC format and partition map (FAT32 with an MBR partition map) and CCC will not recognize it. SuperDuper will not recognize it either BTW.
As was suggested above, your external disk apparently has never been formatted for Mac and is carrying a PC FAT32 format (as MyBook drives are out of the box). It needs to be formatted for use with a Mac for CCC to use it as a Target drive. It needs to be formatted as "Mac OS Extended (Journaled)" with a "GUID" partition map in Disk Utility. Note that this will erase any contents that the drive may currently hold.
Do the following in this order to prepare it:
Select the disk in the left column of Disk Utility
Click the Partition tab at the top of the screen
Change the "Volume Scheme:" via the pulldown menu to "1 partition"
Click the "Options" button at the bottom
Select "GUID" in the window that appears. Click OK to dismiss the window
Give the drive a name and select "Mac OS Extended (Journaled) in the "Volume Information" section
Click "Apply", then click "Partition" in the confirmation window
Wait about 20 seconds or so and the drive will be ready for CCC (or any other Mac use)