It is not possible, with the current installer, to create a RAID1
partition with only one volume. This is not a restriction of the raid-
tools, but that of the installer.
Why would I want to do this?
I need to create a new disk with redhat-6.1 and a raid volume, insert that
into a machine along with another disk (machine only has two drive slots),
copy the data onto the new 6.1 disk, then put a second new disk into the
drive and add the partition on the new disk into the RAID1 set (which will
copy all the data across).
Yes, I know I can do this by creating it initially with two partitions in
the set, then remove one before starting the copy process. But... it /is/
an unnecessary limitation on the installer.
Disk Druid is set up to impose some restrictions on the user to prevent doing
things they would otherwise not want to do. A part of this is stuff like you
describe. You can always use fdisk in order to create whateven partitions you
would like and assign them as you wish.