Replacing the hdd is simple. Depending on which model you have, slim, phat, or superslim, the procedure is slightly different.
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The slim has a single screw at the bottom of the system behind a small, visible compartment. Then you just move the front panel over.
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you'll also have to reinstall the firmware, which goes on there like this: PS3/UPDATE/PS3UPDAT.PUP in all caps. I've never owned a phat to tell you how to replace the hdd, so you'll have to look that up.