Making a RAID-1 Mirror with SoftRAID 4
See my article on how to make a RAID-1 mirror for fault tolerance against drive failure using SoftRAID 4.
Compared to Apple's Disk Utility, SoftRAID has a better interface, notifications and warnings, and does a better job of recovering from failures. It also has a "fast mirror rebuild feature", which can be used for highly-efficient backups ("break" a mirror, then sync it up again later).