I finally got Mandriva installed on RAID1 using two identical 160GB SATA disks, partitioned thus:

500MB /boot  /dev/md0
2GB   swap   /dev/sda2 /dev/sdb2 (i.e. not in RAID)
8GB   /      /dev/md1
149GB /home  /dev/md2
Installed GRUB, the system boots correctly and

cat /proc/mdstat
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?