Indeed, the entire process went without a hitch until the "cp -dpRx / /mnt/md2", when I had two error messages about copying. I wish I had written them down verbatim, but the first was something like:
Copying /rich/something/something/cups/0/certs: Operation not supported
I assumed this might have to do with 'cups' printing and I could safely ignore it. The second was a bit more cryptic and had to do with "/.gvfs". I've been googling like mad about gvfs and learning a bit, though I don't know exactly what might have gone wrong here. The message was "permission denied", so I'm guessing this might be the one.
Anyway, as far a booting back into the normal system, I'm not sure I can! It won't let me sign in. However, I think if I disconnect the 'sdb' hard drive, the 'sda' hard drive should still be in its original state and should let me sign in. I'm not at the machine now, but I'll try it when I can.
Thanks Falko, and thanks for taking the time to provide such a terrific guide!