"The data is all still there and intact on the old hard drive."
Are you sure? If it isn't in that PST-file I'm inclined to say it's nowhere. Are you sure there isn't an 'archive' folder in that PST-file? Make a copy and compact it. How big is it now (in MB): 2 weeks of mail or 1 year of mail?
Or maybe in some other 'personal' pst archive file?