The BIOS remembers what hardware you had, and checks for changes. If it finds changes it updates itself, which takes a few seconds more the first time. It's really no big deal.
I think Rramjet misread the question, there's no need to do anything to your old hard drive if you're doing a fresh install on the brand new one. Even if you're keeping it in the machine as a secondary drive, you can leave it as it is if you want with no problems.