Further to my previous post, there is a KNOWN ISSUE with Seagate 500gb and 1tb 7200.11 drives relating to the firmware. Symptoms include not being detected in BIOS after reboot (without warning) and wrong sizes being indicated in BIOS. Check the Seagate website for more information. If your drive is under warranty, as mine was, you should be able to get it repaired by Seagate (with your data intact) free of charge providing it is the Firmware issue.
Good luck, hope this helps.