It's solved

Walter,

big thank you for your response.

Actually, I got the things right on Friday but it got late and I scheduled calling off the help request for Monday.

Yes, you are right! The thing to do was to install the boot manager on the new drive prior to inserting it as the first-booting one. It dawned on me rather late. A software RAID is different from a hardware one in that: it doesnt duplicate the whole disc but only the parbreastions you specify.

I use grub not lilo and found the way how to install it here: (Somebody had had the same problem before.)

As I mentioned in my post after inserting the the new disc as the third one and adding the parbreastions as spares to the mds

(mdadm --managedev-mdx --adddev-sdcx),

the raids made a sync - no problems with that.

After removing the faulty disc, I was also able to reinsert the rejected sdb parbreastions back into the raids. First you have to remove them from the mds (they are marked as faulty, so this is possible - see man mdadm):

mdadm --managedev-mdx --removedev-sdbx

Then add them again:

mdadm --managedev-mdx --adddev-sdbx

The mds make sync and everything is OK.

I still don't know why they were marked as faulty. Before adding them to the raids, I checked them with badblocks. All were correct. Any ideas?

Thank you again.