Reformatting your hard drive will KILL your FD8 activation. Deactivate your hard drive before you reformat. To deactivate, open Final Draft, go to Help > Deactivate. Follow the onscreen instructions for the on-line or manual steps.
Defragmenting your drive should not affect your activation.
IMPORTANT: Live technical support (phone, chat, email) for Final Draft 8 will end on December 31, 2017. You will still be able to find information and solutions in this knowledge base, in the online troubleshooter and in the FD8 User Guide.