Because raid 1 is mirroring, exactly the same data is on both drives. You should be able to remove one drive and connect it to ATA without problems. You should also still be able to boot the machine. The raid controller will warn you that one drive is not functioning and give you options to enter the raid setup or continue booting. The problem might be that if you modify the data on the drive, then later want it back in the raid, you "could" lose the modified data. If you change raid controllers, be sure to completely uninstall the existing raid drivers before installing drivers for the new controller (unless the drivers are mutually compatible). Some Promise drivers work with a "family" of controllers.