very good help, thanks. So far its looking promising - the device added just fine, and cnd checking /proc/mdstat shows its rebuilding fine
linuxmce@dcerouter:/etc/mdadm$ cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md0 : active raid5 sdb sdc sdd
1953524992 blocks level 5, 64k chunk, algorithm 2 [3/2] [_UU]
[=>...................] recovery = 6.2% (61183728/976762496) finish=216.5min speed=70473K/sec
unused devices: <none>
The big thing now is, we have to figure out exactly why this is happening, as this is a very big important role for the Core (or any server), and I'm sure others will be bit by it.
If you wouldn't mind telling me a little more about what goes on behind the scenes with mdadm (and why it would have removed /dev/sdb in my case), I could then look at including a permanent fix for the 0804 release, as well as improving the web admin tools (for instance, now that I know how to see what drives are part of the RAID and how to add drives, I can add this functionality to the web admin to make it easier for people when they have problems)
Anyways, thanks a lot for your help. I've lost 1.8 TB of information just a few months ago, including irreplaceble family pictures we had just taken in Disney Land due to this same bug. It feels good that I won't be starting all over again!