TO ALL READERS OF THIS THREAD
I know this is an ancient thread, but, to anyone that lands up here lookingfor answers to the 1.5TB drive being seen as 500GB, IGNORE EVERYTHING ON THIS PAGE!!
There is a HARDWARE fault with the 1.5TB drive which causes it to think it is a 500GB drive. As this is at hardware/firmware level, the drive IS actually now only a 500GB drive, which is why it shows up as such in Bios and all other hard disk utility programs.
The problem is only evident with motherboards that have the F3 recovery option in bios (shows up at boot down bottom of screen)
Seagate have released a tool in order to fix the firmware issue. This is the ONLY way to fix the problem.
It is possible that no damage has been done to the file system, depending on what the drive was being used for (boot drive or just data drive) it is however more likely than not, that the original disk partition information and FAT has been lost, so it might be necessary to recover the drive using proper recovery software once the firmware bug has been fixed.