Last time I tried was 11.1, then I wanted to build a degraded RAID1 set, with just 1 md device in it, with view to added mirror later.
The partitioner refused, finding this normal and sensible way of building a mirror pre-loaded with data an error. It wanted 2 RAID devices, for a non-degraded mirror! I think I filed a bug on it, but I can’t remember what happened.
So basically, I would just mount your data on /dev/sdb as a normal filesystem, then convert post-install to using RAID device with mdadm following HOWTOs, then add your
But some ppl are more trusting of software than me, it might work. Though if you look round on forum, number of threads on RAID sets not being recognised I would not bank on it. There’s also a contribution on LWN.net to thread I read (I think on DRBD) by mdraid guy, who explains either block is equally good when they disagree, there’s nothing to say the newest is better when blocks in RAID1 don’t match (which is indeed correct, given possibility of a “crazy” drive/driver writing trash but appearing to function). So the traditional way of syncing up new device to the old, would seem the only safe one.