Janek Kozicki wrote:

Goswin von Brederlow said:     (by the date of Wed, 07 Nov 2007 10:17:51 +0100)

quoted

Strange. That is exactly how I always do it and it always just worked.
mdadm should start syncing on any spare as soon as a disk fails or you
add the spare to a degraded array afaik. No special "start now"
interaction needed.

Thanks for your confirmation. I cannot explain this behaviour - I
just started using mdadm. If anybody here wants, I can remove the
drive and add this again, to see if I can duplicate this "bug" (?).
If so - then tell me what debug information you do need and I will
give it to you.

What kernel and RAID level is this?

If it's RAID 1, I seem to recall there was a relatively recently fixed 
bug for this.

Regards,

Richard