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It's sled up...the tab has to be at the top and tucks under the bottom of the drive. I did this two days ago haha.
 
If you have the MacBook upside down - looking into the battery compartment then the label on the disc will be facing down and the sled will be on the top.

It will go in either way but only work when it is the correct way round...
 
yea i figued it out i was panicing because my ps3 hd needed reformatting and wouldn't from the ps3 because of stuff, so yea...
 
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