[dnanian]

If you want to store things other than a backup on the external, you should definitely partition it. Each partition corresponds to a single source, so if you have three source partitions you'll want four destination ones (the extra one for your other storage).

The general size requirements are that the source and destination partitions should be the same size. But, if you're never going to go over 50GB on one partition, you need not allocate 200GB for it on the backup. Just leave plenty of space, especially if you're going to start up from one: the OS needs space for its own use.