[theed]

I don't mean to sound antagonistic, but I disagree completely w.r.t. speed of software RAID when using RAID 0 or 1. There is so little thinking that needs to occur that it's trivial to do the normal work in software.

As for simply mirroring a drive that already exists ... don't do that. Consider making a RAID array the same thing as formatting. (which it pretty much is) You can't format a disk and retain data. Just like you can't make a RAID set while retaining data. You'll have to move your data off of your drive to configure RAID on that drive. I don't think Apple's utility supports, or should support magical mirroring and RAIDifying a single drive onto another.

You could install the drive and just copy your data over. RAID -1 :-) I recommend that you never run you Hard Drive (main one anyway) at more than half full. If it's more than half full, you need a bigger drive. this keeps you from having to really care about disk and file fragmentation / optimization.