Did you format the Seagate drive before using ? I ran into this problem when I installed a 160 Gig Seagate but after formatting the drive all went well.
I hope you backed up all your documents to CD from My Documents before moving to the Seagate HD.
You might go to
http://www.seagate.com/contact/index.html
Under Technical Support, there is a link you can use for e-mail and get Technical Support.