It looks like the server backup feature was designed for you to plug in an external drive, backup the server, then put the external drive away as a fail-safe. Since the nature of the external drive, you wouldn't want to add it to the server pool.
Now it looks like you can attach a partitioned drive for server backup (one partition might be other files, and other for the server).
But when you go to add a drive to the pool, it reformats it, repartitions it, and shadows it to the D drive, so it really can't co-exist with another partiton (easily).
Since the server information is striped across all of your storage drives (as-it-were), having another partition as a server backup wouldn't help 'cause that drive failing would lose the whole server. The only 'real' way to backup the server is a seperate drive that is not being striped.
Brad
Future WHS user