. . though it may be slow, the array will usually rebuild fine when the bad drive is replaced.

Starting the rebuild, though, can be extremely nerve racking. The RAID controller software is often so bad a normal person has no idea if they are about to write an empty disk over their data or write the data to an empty disk. So far I've never gone the wrong way.

If the controller goes bad you may have no data left by time you learn it's the controller.

I remember a big vendor show (remember those? Bags of swag and an excuse why you must be there instead of at work?) a booth by disk recovery firm Rotating Memories. This was back when hard disks were big and clunky - and veeeeeeeeery expensive. They displayed a high capacity drive they were not able to recover - the owner got so frustrated he shot it with a .38 revolver. Of course it turned out the drive had been fine, it was the controller that wasn't working right.