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.
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.
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!