The problem has long time existed and discussed on APPLE Forum (Since mid 2010). However APPLE not yet react on many complains from their costumers.
The SD card reader slot on iMac/MBP has stopped picking up the SDHC cards. When insert a card, it fails to mount. This used to work fine, and still does if a card is inserted while booting up. But after it has been ejected, it will fail to mount again.
I searched through the entire Internet but could not find information on how to fix the problem and how the SD Reader looking inside.
A year later when I discovered a problem with SD Card Reader in my iMac 27″ (late 2010) I decided to fix it myself.
I’ve ordered the SD Card Reader on Ebay.com and special MAC toolkit.
Follow to Instructions by IfixIt.com , step by step, I have finally fixed long-term problem successfully and myself.
Later I’ve open the bad SD Card Reader and found that problem in the micro contact group (bad in connection). So in my case this is HARDWARE problem. This device is VERY DIFFCULT to repair. Better to buy new one.
See also My Public Photo Gallery
Picture below: SD Card reader without SDHC card inside but the contact group in bad connection (almost not svisible on picture)
Picture below: The SDHC Card insert into SD Card reader slot. The Contact group disconnected.
Description in Russian language
With best regards