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The hard drive in the MacBook Pro is not considered "user-accessible" by Apple, so if you do it yourself and leave any trace, your warranty will be voided. On the other hand, if you have an Apple approved service center do the work, your warranty won't be affected. This can cost around $50 plus the drive, depending on the center, but they probably won't require you to buy the drive from them. The Apple Store will only use Apple-supplied drives, so that means a lot more cost.
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