I can confirm this after installing Ubuntu 10.04.1 server with two 500GB
disks with two RAID1 partitions (1st 20GB for rootfs, the rest for LVM).
The 2nd RAID is incorrectly setup as /dev/sda and /dev/sdb.

Zero'ing the superblock on /dev/sda results in the superblock on
/dev/sda2 going as well.

Item 11 in this obscure Debian mdadm FAQ maybe related, though I did not see 
the same error message:
http://git.debian.org/?p=pkg-mdadm/mdadm.git;a=blob_plain;f=debian/FAQ;hb=HEAD

I think the problem lies with the partitioner in the installer using every 
available KB of space at the end of the disk, resulting in the superblock area 
for /dev/sda and /dev/sda2 overlapping. For details on where the superblock is 
stored see:
    https://raid.wiki.kernel.org/index.php/RAID_superblock_formats

The workaround I used was to end the partitions a few MB from the end of
the disk.

-- 
mdadm misdetects disks instead of partitions
https://bugs.launchpad.net/bugs/599515
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