I’ve had a defective SD card reader on my iMac since I first bought it (a late 2009 model 27″ iMac). I’ve been searching for an answer to this problem off and on for many months. I’ve installed all the latest software updates, zapped PRAM, etc. Nothing would fix it. This morning after a lot of digging, I finally found the solution to the problem.
Problem:
SD card is not recognized when inserted into the slot if the computer is already running, but will be recognized if the card is in place on reboot.
Solution:
Turns out this is not a software problem, but a hardware problem. There is a small switch inside the card reader that signals to the computer that a card has been inserted. This switch can get stuck.
The solution is to insert a toothpick or other non-conductive “stick” into the SD card slot to free up the switch. According to posts on the Apple site, the switch is located at the top/back of the card slot. I also poked around the bottom/back of the slot for safe measure (I never actually saw the switch). After “flossing the inside of the slot a couple times, the reader works once again!

Insert toothpick into slot to "unstick" the switch.

just tried the “stuck switch” toothpick solution on my iMac and it worked. another approach is to insert a compressed air duster plastic tube into the slot for a short burst and poke.
I’ve tried to find a solution on that for a long time! Thanks a lot!
toothpick trick worked great !!!!!
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While this works, what is the permanent fix? I would like to know what can be done to fix the recognition of the SD Card permanently. Does this mean a trip to the tech’s location?
I don’t know of a permanent fix. Since this is a hardware problem, it almost certainly means a trip to the tech.
toothpick worked like a charm, what a sophisticated solution…
you’re my hero!! Thanks.