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

Why do you say that once out of the TC, I might better plug it into a computer, erase and copy .... why would I be able to do this after it is removed when I can't do it now?

Excellent question.

I have come across very few truly dead hard disks from a Time Capsule.

The erase 35 times has clearly failed.. so I assume the disk in your TC is actually bad.. yet even having the ability to start an erase means the disk is not dead dead.. it is unreadable perhaps but I would say in 90% of cases the issues are not the disk but the electronics or power supply in the TC. (I have repaired several hundred.. so it is one area I do claim to know something about).

Plugging it into a computer gives you a chance to see if it is the disk .. and if it is dead.. unreadable then you don't have to worry.. nobody else (without NSA tools and a lot of money/time to waste).. is going to be able to access data.

But I suspect power supply or actual poor board soldering are as much to blame.. The Gen4 did have a reasonable power supply.. although it is now 6 years old or more.. (-2 years in storage perhaps but degradation of electronics is usually time and not just power on hours). A lot of faults .. some very odd ones are caused by under temperature solder flow on the original manufacturer assembly line.. Apple has had more problems in this area than most.. lots of boards and graphic cards they used died and can be resurrected after a solder reflow.

See this .. how to fix the vast majority of dead Gen4 TC is by reflow of the solder.

Gen4 A1409 issues - LaPastenague

It is also I guess a question of just how likely it is that someone is going to get info out of the drive. Apple used to offer a return scheme.. you buy a new TC and hand in your old one.. Apple would dispose of it safely. Mostly of course that means they refurbished them, and resold without giving you anything.. but peace of mind. In Refurb a bad hard disk would be replaced. Some with good SMART data very carefully checked and low level erased. Ask at your local Apple store.. they might still offer some such service.. due to the problem of TC design.. unlike most NAS you cannot replace or access the disk without disassembly. AFAIK the service has stopped.

In my work of repair I often wiped disks on behalf of people who no longer wanted them.. if you have a trusted computer service centre they will also do the job with ease. Computer techs after all carry an enormous amount of trust.. we actually know customers passwords and thus their bank details etc.. plus your browsing history and naughty stuff people get up to.. yet it is really seldom that a Tech betrays that trust.