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It spins for a bit and when the computer realizes that it can't read it, or be able to write to it...it spits it back out or at the very least gives you a pop-up with that information.
 
If you put a disc in upside down...

... call applecare IMMEDIATELY (you did get the extended applecare, didn't you?), schedule an appointment at the Genius Bar!! Take the battery out, then put the entire computer open in a bag of rice for 24 hours (48 to be safe). Then make sure you get new RAM sticks, and definitely upgrade the HDD to a 512gb SSD. Install norton immediately. If you don't have the express card slot, get a dremel and make one so you can get an eSata card for Time Machine Migration (just wedge it in there, it'll fit, eventually).

Then... eh, ran out of steam.
 
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