Recovering from Failing to Mirror Disks on FreeBSD 9.0-RC2

I'm installing a new FreeBSD server, and want to mirror the root disks. According to the instructions in the Handbook and my own Absolute FreeBSD, it's a simple process. The instructions are not valid for FreeBSD 9, however. It was late. I was tired. I tried anyway.

The first clue should have been that the disk devices now have different names. Rather than /dev/da0s1, they now look like /dev/da0p1. What difference does a letter make? Well, my test instance is virtualized. I took a snapshot...