Can you use drives while Raid 1 is rebuilding

Doesn't sound good...You can try ripping the "rebuilding" drive out, on the chance that you just got a DOA drive, but chances are it was one of the others that failed. Rebuilding a failed drive is a common source of failure with raids...It's an intensive process.

I once had a 20 drive array fail one drive after another as the array was being rebuilt; when the smoke cleared I'd replaced 18 drives because they warned of failure, and 2 others just because I didn't trust 'em (This is why you never buy 20 drives from the same production group).

IO time will be consumed by the operations which will be time not available for the rebuild, slowing things down slightly, but it's definitely safe to write to the array while rebuilding.