Yesterday one of my trusty old SD cards broke in a really simple way. The lock tab fell off. I immediately assumed the lock tab was some kind of tiny switch that jumpers tabs on the SD card, so I peeled away the plastic shell to look inside.
Turns out the SD card write lock mechanism is on the readers, not the SD card. Fixing the lock mechanism (or rather, permanently setting the card to write allowed) is as simple as putting something in/over the lock to hit the switch on the reader.
I superglued the card back together with an additional tiny piece of plastic and tested it and it worked, with one caveat. The push-inward-to-eject feature in my camera's SD card slot wasn't working, presumably because the card was too thick. This meant that to take it out, I had to remove the camera battery, then carefully pry it out (there isn't much room so it is tricky). Not happy to leave things half done, I sanded it down until it fit.
Of course now neither my cheap SD card USB reader, nor my camera, could read the card. The push to eject feature worked smoothly, but that was no help for a card that wasn't recognized.
So now I shimmed the card with a piece of cardstock, and the USB reader read it fine. Now I tried it on the camera. It was not read by the camera, nor did the push to eject feature work smoothly anymore. Darn.
Still not content, I reinspected the card, and discovered that the most likely culprit of the push to eject failure was actually the replacement lock. There's a groove in the side of the card and my replacement piece of plastic for the lock tab interrupted that. This is what was catching and preventing the mechanism from working. So I tried to clean up my work with an x-acto knife. Results: Still not clean enough for the push to eject mechanism, and now the USB reader thinks the card is locked again (the tab isn't hitting the lever anymore since I removed some material).
Okay moral of the story: 'the enemy of good is better' and more importantly, 'SD cards need to be very precisely the right shape to work in many readers.'