Disassembling the Xbox 360 HDD unit
Start off by first removing the HDD unit from the Xbox 360 main console and lay it flat with the connector side facing upwards. You will see 3 screws. The 4th screw is located underneath the silver "Microsoft" sticker -- removing this sticker instantly voids your HDD unit's warranty.
Once you have removed all 4 screws, lift up the plastic cover while being careful that it is latched onto the button-release end of the unit. You'll want to be careful as not to pop the spring loaded button and lose the tiny spring.
Once open, you will see the 2.5" Serial ATA notebook hard drive encased in a metal shell. Lift off the cover of the protective shell by first removing the four black screws holding it down. To proceed further you will need to have a thin knife. The metal casing is attached to the plastic shell by a very strong adhesive, and to remove the actual hard drive from the housing, you must first pry off the plastic shell from the metallic HD casing. Simply insert a thin knife and slowly "saw" away at the adhesive. Shortly after you will be left with the following:
Once this is done, you can simply remove the attached Serial ATA data cable from the HD and slide out the drive.
Credit goes to TTG.