Well, if you really want to mirror, you can do that using hardware mirroring (with a raid controller), or you can do software mirroring (through XP or other OS that supports it).
Unfortunately, "on the fly" mirroring has one bad side effect: if you erase something by accident, or a virus destroys some data, you have no backup, because the data will be destroyed/erased on BOTH copies "on the fly".
For me, a better option is to hook up both drives, then use a program like "SmartBackup" version 1.64. It will let you create "backup projects" (basically, scheduled backup jobs) that are highly customizable and configurable such that you can grab specific info for backup without backing up useless junk. It even allows you to use variables and such in naming directories, so you can use dates in the names of folders to keep various backups without overwiting other backups. This comes in really handy if you want to make backups of programming work as you go along -- at some point you might want to go back to a previous version, but if you just overwrite your backup all the time, you lose "history" of your changes.
That's my .02
