micro SD dentro il vano per I CD/DVD

I just helped out a friend of mine with the same problem last week. Try folding a paper sheet in the shape of a ruler and see if you can take it out by gently reaching inside the CD slot. If the card is too deep in you will have to take the Optical Drive out and shake it up a little while holding the CD slot a bit open with your fingers... The SD card will drop out easily. Here is the link to the guide to take out the optical drive.

Reemplazo del disco óptico del iMac Intel 21.5" EMC 2308

Hope this helps! good luck!