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.
Problem description:
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...