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You could probably use cardboard to make up the difference. And yes, it is important that the SSD be secure. If it's not secured and can move, it can cause stress on the cable which will lead to it breaking. I don't know what the internals of the MBP look like, but I do speak from personal experience with a different laptop.
 
yea I would like to +1 on this thread... not to be a bumper... but I found a intel x25 160gb for 315 on ebay but it is the 7mm version... so what do I do to install it... will it make ANY difference.. detailed explaination would be awesome as I have never installed one before and I dont want any cardboard inside my brand new 2011 2500 dollar macbook pro... that seems ridiculous.
 
Try to find some durable material (rubber?) that can be cut to 2.5mm to make up the difference. If not, something that doesn't absorb heat like rubber, but I doubt it would get hot enough to be a problem anyway.
 
You don't need a spacer; the drive is held in via screws on the side.

If you did need a space you'd like a 5.5mm spacer.
 
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