I can't find it, but I read an article/blog/post somewhere that says time capsule cannot be partitioned except through some sort of hack. It may have involved removing the hard drive, mounting it directly, partitioning, and then reinstalling it. But I looked more than a month ago, so someone may hae gotten a better answer since then.
ETA: Never mind--you're asking about a normal external, not time capsule. Apple should have named time capsule "time machine", since it's a machine (of sorts). They could have named time machine something else (flux capacitor?)