usually you make sure the other disks in the array are ok, then you swap out the disk and insert the new one.
but now, once the new disk is already in, you may make it a spare, that will kick in once the disk is bad or degraded, depends on your settings.
but now, once the new disk is already in, you may make it a spare, that will kick in once the disk is bad or degraded, depends on your settings.