Christopher> Pulling one out is ok. We had some problems when
Christopher> inserting a disk that was used as half a mirror
Christopher> before. This actually killed the machine. The disk had to
Christopher> be cleaned before reinserting.

We've also encountered this problem.

We've never had a problem once we "clean" the first handfull of blocks
on the disk. We've never done any testing to see exactly how much
needs to be cleaned. What we do is mount the disk in another box, run 
dd if=/dev/zero of=/dev/DEV count=1024 and then the controller always
sees it as a new disk.