I keep 4 partitions on my internal...

1. My main bootable volume.
2. My music, docs, and 3rd party apps & installers.
3. A clone of my main volume.
4. An empty spare.

That second partition saves me an enormous amount of time in the event I either need to or want to reinstall OS X.

Yeah, sizing the partitions has gotten me into trouble in the past, but with the size of today's HDDs, it's a snap.

The new Great Equalizer is the SEND button.

In Memory of Harv: Those who can make you believe absurdities can make you commit atrocities. ~Voltaire