Can I install OS X onto a separate external drive partition? There are plenty of articles explaining how to install OS X on a separate partition of your Mac’s internal drive and how to install it onto an external drive using a single partition. However, it’s not clear to me whether it’s possible to install OS X onto a separate partition of an external drive. It’s what I’d like to do, is it possible?

For what it’s worth, my external drive currently has a single 500GB partition, is formatted as “Mac OS Extended (Journaled)” and has a GUID Partition Table. That should make it bootable, right?
Yes!

As the disk is the correct format, it will show in the disk list in the installer (click Show All Disks). You can install any version of OS X on a partition of the correct format, regardless of where the partition is located.

The OS X installer doesn't partition the disk for you though; do this first in Disk Utility.