(Or: Everything you wanted to know about multiple hard drives, disk partitions, and removable volumes, but were afraid to ask.)
Although most Macs come with a single hard drive, pretty much every Mac user will at some point work with multiple mounted volumes—the technical term for hard drives, removable media, and even disk images. In addition, many users partition their hard drive(s), which is a way of making a single drive appear as multiple drives. Although OS X, like any modern operating system, fully supports as many volumes as you can manage to mount, working with non-boot volumes is a bit different than working with the boot volume. In fact, although the Mac OS has always been a bit confusing when it comes to working with multiple volumes—especially for new users—OS X has additional considerations that make multiple volumes a bit more challenging.
In this appendix I'm going to talk about some of the issues and quirks that arise when working with multiple volumes. I'm also going to talk about drive partitioning, including why you would want to do it, and how to actually go about it. Finally, I'll discuss a couple of ways to customize OS X when using multiple volumes, by moving user directories and/or your system swap files to non-boot volumes.