You need:
1. CarbonCopyCloner version 3.4.7. You can download it from this page:
http://www.bombich.com/download.html
(Important: version 3.4.7 may throw a warning up if you use it with Mountain Lion that it "hasn't been tested" with ML. You can IGNORE this -- just dismiss the alert, close any web windows, and continue with CCC. It will work fine)
2. A drive volume large enough to hold the contents of your MacBook internal drive.
You could take the (as yet-unused) WD drive and do this:
1. Initialize with Disk Utility. Give it a distinct name.
2. Use CCC to clone the contents of your MacBook to the WD drive.
When done, you can "test boot" the WD drive by doing this:
1. Restart
2. As soon as you hear the startup sound, hold down the option key and keep holding it down -- the startup manager will appear
3. Select the WD drive using the pointer or arrow keys, then hit return
4. The MacBook should boot from the backup
Check "About this Mac" to be sure you're booted from the backup. You just want to be sure the backup clone works as intended.
At this point, you can install ML onto the MacBook.
If it was me, and I _KNEW_ I could boot from my cloned backup, I would do this:
1. Reintialize the MacBook drive
2. Do a completely "clean install" of ML to the internal drive (but don't use Setup Assistant to migrate from backup drive)
3. Boot from the fresh install, create a "blank administrative" account, run software update to get the OS up to date.
4. Then, I would connect my cloned backup, and use Migration Assistant to migrate over my accounts, user settings, apps, and data.
5. Then, I would "switch accounts" to my original (my named) account, and leave the "Administrative" account there, untouched, for emergencies, etc.