[dnanian]

Hi, Mary.

I'm glad you caught your "USB Mistake" -- definitely switch that for a boot-compatible FireWire drive (see the User's Guide for some recommendations).

People partition their external drives so they can use them for backups and other things as well. For example, if you have two internal drives you want to back up, you'd want to create at least two partitions (volumes) on the external, one for each internal.

If you have extra space on the external, and you also want to store data on it, you can create a 3rd partition.

It's very easy to do using Disk Utility: just choose the drive in the sidebar, then click the partition tab, and the rest is pretty clear. The only trick is that you're going to want to make sure the "Partitioning Scheme" is "APM" -- Apple Partition Map. That's in "Options" in the Partition tab.

Hope that helps.