Old Time Capsule not working, how can I destroy data?

I understand that when unplugged (2 years), and then plugged in, Airport Time Capsule will self-diagnose and repair itself and that other than that there is nothing I can do to repair the disk

That's the theory. When the Time Capsule is powered up, it automatically goes through a number of checks to fix any software problems that might exist on the drive.

If the disk.....hardware..... is physically damaged or failing......then your only option would be to physically remove the drive and destroy it.

Try to erase the drive first, so see if that can be accomplished. A Zero Out Data secure erase will take 5-8 hours or so depending on how large the hard drive might be. Try that option, then check to see if the Time Capsule is operational again. In other words, if the error messages go away, the Time Capsule is still good.....although 3-4 years is about the average useful life of a Time Capsule.

A 7 Pass Erase will Zero Out the drive 7 times, so it will take 7 times as long as the Zero Out Data option. Use the 7 Pass Erase if you are the really insecure type that tends to believe in most conspiracy theories.

A 35 Pass Erase will Zero Out the drive 35 times, so that might take weeks. No reason to do this unless you are storing nuclear codes or other things that would interest the NSA.

If you want to destroy the drive, then there is no reason to waste time erasing it. Pull the drive out of the Time Capsule and crush it in a vise or give it some good hammer blows. This would assume that you still want to try to use the Time Capsule as a WiFi router......like an AirPort Extreme. If you don't want to do that, then just crush the entire Time Capsule in a vise.