Open Disk Utility and select the external harddrive in the left column. Select the "Partition" tab. Select "1 Partition" as the Volume Scheme. You can give the volume a name now if you like, but you can always change it later. Choose "Mac OS Extended (Journaled)" as the Format.
Click on the "Options..." button and choose the "GUID Partition Table". Click "OK" and then "Apply".
With the external harddrive plugged in the USB port, run SuperDuper! Select: Copy "Macintosh HD" to "500GBHD" (or whatever it's named in your case) using "Backup - all files" and click the "Copy Now" button. After an hour or so depending on how much data you have, the external harddrive will be an exact copy of the harddrive inside your MacBook.
Once the cloning is complete you can do a test and boot from the newly created bootable image on the external harddrive by restarting your MacBook and holding the option key. This will let you choose which disk you'd like to boot from. You should be able to boot from the external harddrive at this point and it will look like you booted from the internal harddrive that you just cloned.
Disassemble the external hardrive enclosure. The construction of harddrives will vary, but whether it's held together by screws or just snapped together, you can probably get it apart one way or another. For the Western Digital My Passport Essential, you need to insert your fingernail or a knife between the top an bottom faces and the long side and pop the pieces apart a bit.
You need to loosen all the snaps all along the long edge, BUT NOT on the shorter edges. After the long edge is loose, gently slide the inner part out of the enclosure. I found it a bit difficult to get the sliding started so I helped it along with by twisting the tip of a flat head screw driver between the inner part and the edge of the cover. As with most things in life, having patience and not forcing things is the key to achieving your goal.
Next, carefully remove the plastic piece encircling the harddrive noting how all the pieces fit together so you'll be able to reconstruct everything with the old harddrive later.
Slide the harddrive out of the metal sheath.
Very very carefully detach the USB connector from the SATA power and data terminals on the harddrive.
You might want to use a thin blade and pry apart the connection right where they mate rather than torquing on the USB PCB with your fingers.
Once the connection is opened a little bit with the knife, insert a flat-head screw driver and twist to pry the connection apart at different locations along the connection.
After the connection is almost separated from the twisting screwdriver method, grasp the USB to SATA connector PCB by the edge and gently wiggle and pull apart the connection.
Here's what the SATA data and power connections look like. Mini-IDE connections look different.
To reconnect the USB to SATA PCB to the hard drive, get the connection started and then use your fingernails on the edge of the connector. Do this rather than pushing on the fragile PCB.
Remove the battery on the bottom of the laptop using a coin to unlock it.
With the laptop flipped upside down and the battery removed, unscrew three little screws using a small Phillips head screw driver that hold a retaining clip. You don't have to fully remove the screws from the retaining clip to remove it.
Once the retaining clip is removed, fish out the white plastic flap and pull to slide out the old harddrive.
With the old harddrive out, remove the metal sheath from it by taking out the four screws. The screws are kinda funky because they require a star-tipped screwdriver. I just used a pliers to grab onto the screw heads and twist them out.
