simple answer is no you don't need to partition the external hard drive and personally i never bother with external backup drives.
main reason to partition a drive is to keep the operating system separate from your data, that way you can reinstall the operating system and not lose your data, of course that's not the only reason to have multiple partitions but it's the only one that really matters in my opinion.
so seeing how there is no operating system on an external hard drive i see no reason whatsoever to have multiply partitions on an external data/backup drive.
of course there is no reason why you can't create partitions on an external drive to maybe help you better organise your data but personally i see no benefits because separate folders work just as well to do that.
if the hard drive fails, having separate partitions will not make any difference security wise than having 1 single partition. you will still lose all data on the drive either way.
here's some links that you may find of interest/help. :-