Forcing a drive online does not make any changes by itself - it simply tells the controller to load and use the configuration on that disk.  If we were talking about a distributed RAID array where one disk only had part of the total data (like RAID 5), then we would not want to do this, as the controller would attempt to reconcile data on all the drives, and if any of it didn't match, it would simply - and probably arbitrarily - choose which disk to use, using a mix and scrambling your data.  In a RAID 1, each disk contains a full copy of all data on the array, so you can force online one drive without affecting the data on the other drive.  Forcing online both drives at once would likely be a disaster, but one at a time is fine.  Forcing online one drive at a time will not affect a data recovery pro's ability to recover data, should you need to go that route ... many are capable of recovering data from even the "disaster" scenarios I described, but there is no need to risk that here.
My PE2850 has a PERC 4/DC controller with two U320 SCSI disk in a RAID1 on Channel0 to run the OS. Both disks are marked as failed (E0D76 message on the front display), and the OS fails to boot. Channel 1 on the controller has a RAID5 with 4 disks, which appear to be fine (green lights,"optimal"). When I replaced the two disks on channel0 with brand new ones, they are still marked as "Fail". Is it possible (likely?) that one of the channels of the RAID controller is broken? If so, shoudl I just get a new PERC4/DC card and put it in? Or does anyone have any other ideas? Thanks, Bart    
RAID Controller (PERC 4/DC) fails on single channel in PE2850 ?