Because most external hard-disks are usb the drive-letter sometimes changes. When I give for example drive x to back-up and I have switched some usb hardware the hard-disk gets a different letter and back-up does not get executed.
That’s because the backup “set” you created has hard-coded drive letters in it.
It would be the best to make sure your external device is mapped to the same drive letter before backup.
Now i think i read somewhere that there is a tool to prevent drive letters from changing, you can set them prefixed so once it detects the newly connected device it will always have the same letter, provided there is no other connection to the letter (mapped drive etc).
I have no experience with this tool but maybe this could do the trick:
In order to prevent that problem, FreeFileSync (http://freefilesync.sourceforge.net/) has an option to use volume labels instead of drive letters. Could CB do this in future versions?