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Upgrading the HDs will Void your Warranty, if you attempt to do this yourself, since this is not as easy. Also, Apple specifies that if you attempt to make an AppleCare claim and find a non-Apple certified HD, they can refuse service.

I know what your saying, but on my MBP, when i sent it in for them to Repair my IR port on my laptop, they refused because the prior owner changed the HD to a non Apple Certified drive, which does work. I'm on it right now.

The previous MBP's hard drive wasn't user-serviceable. But on the plastic MB and all the unibodies, the HDD and RAM are user-serviceable (except for the Air).
 
HERE are the instructions published by Apple for a consumer to replace their own Hard Drive. It warns to follow the directions or it is possible that you will void your warranty. This means that if the procedure is carried out per the instructions, your warranty will not be effected.

Also, I've taken my MacBook to the Genius Bar before and told them about upgrades to both my RAM and HDD and they were good with it. One visit, it was determined that there was a logic board failure and I had installed upgrades in place. I really don't think that you need to stick your old stuff back in but I'd at least make them aware of any upgrades up front.
 
Seems Like Apple Is Never Consistent With Their Warranty Decisions

I have a MacBook late 2008 unibody with upgraded WD 500 HD sent in for hinge problem, display works fine just is awkward opening and closing it....

The repair was held up when received and I got an email stating I had third party hardware, the WD 500, and it would cost me 700 dollars to put original apple part in.

I called, complained and they maintained their position.

So am having it sent back and will put original HD back in and send it right back hoping they have not just completly voided my AppleCare warr.

They really stuck it to me. Imagine paying 700 for a 200 HD!

A lot of nerve these people have and what recourse does one have, NONE!!
 
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