In the introduction you should link fixers to this excellent doc: https://www.ifixit.com/Misc/HD_Software_...
It is really critical, super easy, and free(!) to clone your existing drive onto the new one you will install. I ran into one error, but SuperDuper! support replied immediately on how to fix it...Thanks ifixit and SuperDuper! (I ponied up the $28 for the software anyway, I was so impressed!)