According to http://www.rakuten.com/prod/120gb-hard-d..., your hard drive is the right size. If it doesn't fit, check to see if the pins connect correctly and if your laptop accepts a SATA drive and not a PATA drive; this image can help http://www.laptopking.com/sata_identify.....

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Next, check if you're inserting it correctly. I haven't experienced replacing the hard drive with the M55-S1001, but I assume it's the same method as the M55-S135 (though it could be different).

I'm unsure if there's anything else you would need besides the correct type of screwdriver to unlock the casing but I doubt it.