quoted

If the / was on a RAID5, would it be able to boot with 2 disks ?
If so, is it possible to convert my RAID0 to a RAID5 ?
For example, I could boot on a CD, backup / onto /home, delete the RAID0 
array and recreate it as RAID5, then restore the backup. Would this work ?

    

Yes, a RAID-5 will run fine with just two disks. You could certainly
boot from CD, backup, recreate the RAID, then restore - you'd probably
need to update the mdadm.conf file (and the initrd) to indicate the new
array UUID and details though.

You may also want to read through the GRUB section on
http://en.gentoo-wiki.com/wiki/RAID/Software to see how to install grub
onto all the drives (avoiding the issue of which drive is currently
being booted from).

  

Ok. This is the configuration I had :
/dev/md1 was a RAID0 composed of /dev/sda2, /dev/sdb2 and /dev/sdc2

I did this :
mdadm --stop /dev/md1
mdadm --zero-superblock /dev/sda2
mdadm --zero-superblock /dev/sdb2
mdadm --zero-superblock /dev/sdc2

and then this :
mdadm --create --verbose /dev/md1 --level=5 --raid-devices=3 /dev/sda2 
/dev/sdb2 /dev/sdc2

I used the rescue mode from an alternate CD (hardy 8.04.2) to format 
/dev/md1 as ext3. Then, I restored the backup and updated the 
/etc/mdadm/mdadm.conf file with the output of mdadm --detail --scan.

Now, GRUB loads (it was on /dev/md0 which is a RAID1 composed of sd*1), 
and it tries to load Ubuntu, but it just won't load.

You said that I would have to edit initrd... what is that and what do I 
have to change in it ?

Thanks,

Maxime Boissonneault