Tim Bray - The Databox:

"Occasionally, one of the disks might fail. When this happens, you won’t lose any data, but a red light on the Databox will start flashing, and it will send mail to a few designated addresses. When this happens, it’s exactly like when your laser printer starts saying “You need to replace the cyan ink” or “You need to buy a new printer drum”; next time you go shopping, you swing by Best Buy or Costco and pick up another disk unit. When you get home, you open the top of the Databox, pull out the disk with the red LED turned on, drop in the new one, and toss out the old one."