Or instead it might go like this -
1) Turn off - swap one for a new terabyte (or more) disk
2) Turn on - writes the old disk as a blank mirror copy of the new disk
3) Turn off - swap other disk to a new terabyte (or more) disk
4) Turn on - writes that disk as mirror copy of the recently written blank disk
5) retire to pub and ask about restoring from backup
Sorry, I have no solutions, only questions:-)
I have always been afraid of disk mirroring (etc...)