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I had this happen to me and wanted to do a write up in case anyone else is on the internet frantically searching for an answer to this terrifying event. The SD slot on the new iMac`s is directly below the cd drive, and since it is on the side of the computer, if you are not looking, it is easy to slip an SD card into the cd drive. I did this and the last thing I wanted to do was lug my 20 pound computer through a crowded mall to the Apple store and wait in line for an hour. I gathered different ideas from other people and this is how I solved it:
1) Calm down - it will probably take about 30 minutes and stressful moments where you are sure it will never come out, but stay calm, collected and determined to get it out. You most likely won`t break your computer, unless you are super rough about getting it out.
2) Turn off your computer and unplug it.
3) Make a tool like the photo shows. I used a hard plastic rewards card. Do NOT use something as thin as paper, because it will tear and could get stuck in there with the SD card.
4) If fishing it out doesn`t work, then get a towel and carefully lay your computer face down on the towel. Try fishing it out in this position. Shake your computer around a little to try to figure out where the card is and fish in that direction.
5) Hopefully this should work, but if it doesn`t, unfortunately that means a trip to the Apple store! Good luck!
Pictures are mine.