I don't usually defrag my backup drives since I don't often delete my backup files and then replace them so fragmentation is not a serious concern. Even so, a backup drive with semi regular deletions is not nearly as likely to be fragmented as the primary OS drive which is constantly adding, creating and deleting files and is also host to the pagefile, hibernation file and Master File Table each of which is large, dynamic and can add to general fragmentation.

As far as partitioning a backup drive it's not generally necessary and I'd only do it for organizational reasons.