I have a Seagate 250 GB SATA (7200.8) that all of a sudden is not recognized by windows anymore. It was/has been working fine and now windows says its unformatted. WTF?
download Seagate Dianogistic Tools and burn them to disc. Run the Tools before Windows loads and see what the outcome is of a full read check. This takes awhile. If all signs are good it could be an issue with Windows.