Nowadays, many storage systems utilize a hot spare disk which is dedicated to save your ass if a disk fails. But a few people really understand how to handle hotspare disks correctly - these disks required to be checked periodically.
Let's consider such a situation - you create your own RAID and put N drives (along with the hotspare disk) to the array. In this case there exists 1/N probability that exactly hotspare disk will be the first to fail. So now you no longer have a hot spare.
When one of the RAID member disks fails you are surprised to learn that the hot spare disk already failed.
To avoid this you need to check a hotspare periodically or stick to RAID6E or RAID 5E/EE layout.
