A maddening little mini (and I mean that literally) crisis earlier sent me scurrying onto the Internet in search of solutions. An SD memory card flat out refused to work properly, as the camera kept refusing to take pictures:

“MEMORY CARD LOCKED”

was the irritating error code. Looking closely, I saw that there was no sliding memory lock tab, which had borken or fallen off. Hardly surprising, as this is one of those SD sticks which hinges in the middle so when you fold it, it doubles as a USB memory stick. Probably, the whole thing is cheaply made, for the tiny plastic slider is too close to the hinge, and it must have fallen off during one of the insertions.
There was no finding it and sticking it back in, as these things are barely larger than a grain of sand. I nearly drove myself crazy pulling another teensy slider lock out of a freebie 16MB card which came with my camera and will never be used, and imagine my chagrin when it didn’t fit a card from another manufacturer.
Initially, I thought the switch was actually a switch that did something inside the card. Not so; it turns out that it’s just an indicator akin to the tabs in VHS cassettes, which switches an actual switch inside the camera. The latter looks for the correct gap and if it finds it, the SD card “locked”; if it doesn’t find it, it’s unlocked:

“The switch / notch works in same way as the notches on compact audio cassettes and videotape cassette tapes or floppy disks. A closed or covered notch is writable, while an open notch (or removed tab) is protected.
If the switch becomes broken or falls off then the card will become a write-protected ROM card and no longer be writable. A possible troubleshooting solution would be to apply tape over the notched area (avoiding the connectors and the other notch) to configure the card in a permanent writable state.”

The old “Scotch tape” cure solved the problem for me!
Here it is; larger than life:
SDlock.jpg