SD, my RAID drives will be the exact same models of HD, Seagate 160gb SATAs, which I purchased at the same time. My new, third HD is also a Seagate 160gb SATA, but a slightly newer version.
Jack, mirorfolder looks good. According to their web site, mirrorfolder should be able to function very similar to RAID 1. I don't know enough about RAID 1 or mirrorfolder, however, to determine if one is better for my purposes than the other. I'll investigate further, while also pursuing RAID 1.
With three HDs, only one of which contains any data, I suspect that there must be a realtively simple way to end up with RAID 1 with a mirror of my existing drive. For example, maybe I could copy virtually all of my existing data to my new drive, set up my two old HDs as a clean install of my operating system in RAID 1, then copy all my old data from my new HD back into the new RAID 1. I just don't know enough about computers to know if that idea will work.
I tried to copy my old "C" drive onto my new HD using Seagate Disc Wizzard 2003. All but six files were copied, which prevents me from using my new HD to boot. I assume that those six files are related to my OS and couldn't be copied while my OS was in use. If so, then maybe my idea described above would work. Or, maybe there are better solutions. I know that there are a lot of computer experts on this forum and hope to hear from them.