Carson said:
I've tryed 5 or so, none of them work.
If you have tried a different brand, a movie dvd, then try to restart the iMac and see if it still does it. If it does, time to call Apple up and take it in. I had an iMac that had a SD DOA and did the same thing. Even though yours did work there is no set amount of time that a SD will work for and just one day it has had enough
Good luck and post back if any of the things mentioned don't work.