The lock tab doesn't actually do anything to the card per se. It's just a piece of plastic that slides on the edge of the card (not electronic in any way).
It's the card slot itself that detects the position of the plastic lock tab. Typically there's a tiny micro-switch in the card slot which detects if the tab is slid in the lock vs. unlock position. If that micro-switch is jammed or blocked in some way then it's possible the camera "believes" the lock tab is in the locked position, even it is unlocked.
Inspect the card to make sure it's not in the "locked" position and if it still refuses to format in the camera then check the slot to see if there is anything stuck in the slot.
I have heard of people inserting cards backwards, breaking some of those fine plastic fins between each of the gold contacts, then then those getting left behind in the slot and jamming things up. But if the debris is cleaned out, the slot and cards work fine again.
If that doesn't do it, the camera may need service.
Tim Campbell
5D III, 5D IV, 60Da
