Q: What is the default behavior when a drive fails?
A: The default settings for a MegaRAID controller will enforce an automatic rebuild policy, and usually an automatic copyback as well. The automatic rebuild, in the presence of commissioned hot spares, will automatically set a failed drive offline, and designate the first compatible hot spare attached to the controller to take the place of the failed drive. At that point, you need only remove the failed drive, and replace it with a spare. The controller then copies the data from the previously consumed hot spare back to the new drive you inserted, and recommissions the hot spare to be used for the next failure.
For safety and security, LSI MegaRAID controllers maintain an exact layout that maps drive metadata by enclosure and slot number, so hot spares are always temporary. Moreover, drives are not allowed to move to different slots, nor are they allowed to drop offline without warning.
Any drives that are removed from the enclosure for any reason are marked as missing, and cannot be imported back into the array without manual user intervention. This behavior is designed to prevent faulty drives from randomly disconnecting and reconnecting, which may corrupt the data on the rest of the array.