Do you have a backup of your old hard drive?
Basically iTunes thinks that the iPad has never been connected to the computer before since it doesn't have records of sync or backups. Therefore if you want to associate the iPad with the freshly installed OS, you need certain files from the old machine otherwise you have to wipe the iPad and start fresh or restore from iCloud. Do you have a time machine backup?
I've been dealing with this same thing with my mother who bought her first new Mac in 7 years. We did a clean install/start from scratch setup bc migration assistant has always had problems transferring apps/data from a PPC Mac to a Intel one. It seems that iTunes backups are done differently too.
The geniuses at the apple store didn't know the answer. I'm working on it, maybe I can help if you give more info, about any backups.