My mistake.
However I admit to being allergic to partitioning because you almost invariably end up with the space you desperately need in the other partition not to mention repartitioning often does not work as well as we might like when it becomes necessary.
Time Machine backups are sparse image bundles which are as effective as partitions in separating different data sets, but have the distinct advantage of being able to grow or shrink on demand. The only limit on their growth is total drive capacity. The only disadvantage of sparse image bundles in the scenario you described is that like any dmg they don't mount automatically and have to be explicitly opened. They are worth considering as an arguable superior alternative to partitioning.
