Depending on the the actual raid card, it may be possible. However if it is a local host raid imbedded on the MOBO you may not be able to do to. You an P to V your raid system to another array with identical partition.
Unfortunately metadata (the stuff used when accessing an array telling the OS what the disks' layouts) is not standardized and is written on the disks themselves in host-based RAIDs. That being said, it is completely possible if you stay within the same copmany and, more precisely, the same controller model, to "split the mirror" and rebuild from that.
Before doing anything, however, it is advisable to backup important data or even mirror/ghost the entire array if possible.
I've seen compatibilty go either way with RAID1, it depends on where the metadata for rebuilding is put. Many do indeep put it at the "end" of the array and don't mark it in the partition table, meaning that the disks are indeed useable to regular systems. I have also seen instances where the controller/software's done something wonky with that metadata, causing errors on boot. This is definately and area where you need to see what is on your motherboard and do some research (or back things up and throw caution to the wind).
But you are correct, Codesmith, if there are no issues reading the disk, you would not reinitialize the array via BIOS, instead you'd first enable them for RAID-ability, then use whatever tools came with the motherboard to rebuild the array or, more likely since the controller won't recognize the disk as part of an old array due to incompatible metadata, using those tools to create a new array if and only if the tools allow creation with backing up the contents of one drive to the other (as NVRAID's do)