Reinserting the card without the lock piece results in it being read as write-locked, so unusable. (Some background: the write lock is just a piece of plastic that touches a switch inside the reader slot. Some readers ignore them. However the lock piece must be present in the right place for the card to be recognized as writable; if it's missing entirely, it defaults to locked)
Inspecting the card, I noticed a scratch down one edge of it that goes right over the base of the write lock slider. I took a close look in the SD card slot and I see a fairly sharp tab of metal sticking out that is consistent with the scratch on my card. In fact the way this tab drags across the card and write lock, I would expect it to push the tab to the locked position when inserting a card, and to push it back to the unlocked position when removing it. This seems nuts. Here is a photo in which the really shiny thing is that tab. Has anyone seen this before?
I am trying to decide if there is something more prudent to do than to stick a small flathead screwdriver in there and flatten out that tab. Conceivably it may have started off not bent to stick out at a 90 degree angle, but instead at a shallow angle so that it would only catch on the card when you go to remove it for the first time, which would bend it out to create the problem of damaging the write lock sliders. Why it's there at all is not clear to me.
I really hope to avoid taking the whole machine apart to sort out the CF reader slot. And seeing that the reader slot is made out of sheet metal, and the 30% 1-star reviews for FlashAir on Amazon, I can only imagine that sticking a WiFi device inside a sheet metal box is not going to work out, so I don't see the FlashAir or equivalent product as a solution to a CF slot that destroys cards.