I ran into trouble and decided to go another way.
The rescue CD did not find CentOS installations, most probably because the ext3 partitions were hidden behind a md layer.
I could have stopped the raid devices first, then I would probably have been able to use the rescue CD and rebuild initrd & install grub.
However, as I already have one disc up (with a fresh CentOS installation), I decided to rescue my data from the "old disc" (the non-booting one) to this "new disc". That way I will get a working system.
- Jussi