Here's the IFIXIT guide: [guide|47457|Toshiba Satellite P55W Hard Drive Replacement]
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But before you sip your current drive out (thats if it's working), you'll want to clone your stuff over using a SATA to USB adapter cable like this one: [https://www.startech.com/HDD/Adapters/USB-3-SATA-adapter-cable-with-UASP~USB3S2SAT3CB|Startech 2.5" SATA to USB adapter]