Comment 3 for bug 1080901

it seems mapdevfs (or grub-installer, depending on your point of view, i guess) can't handle filesystems spanned across several devices. (I also can't figure what mapdevfs is supposed to do, in my tests, it just returns its own input)

Workaround was to set up btrfs on one disk and add the second after install, followed by a btrfs fs balance.