"Incorrect Superblock" with one disk missing from RAID
I finally got Mandriva installed on RAID1 using two identical 160GB SATA disks, partitioned thus:
Installed GRUB, the system boots correctly and
500MB /boot /dev/md0
2GB swap /dev/sda2 /dev/sdb2 (i.e. not in RAID)
8GB / /dev/md1
149GB /home /dev/md2
shows the RAID is up and running.
Before I move my data over to this machine I did some testing. Powered down, disconnected the first HDD (/dev/sda) and restarted.
The GRUB menu appeared and the system booted, but when it got to checking the filesystems it complained about "Incorrect Superblock" on all 3 RAID devices (md0, md1, md2) and halted.
Re-connected the first HDD and it booted normally.
Repeated the above with the same results.
What have I done wrong and is the RAID actually working properly?