You could check the Seagate site to see if there is a firmware update for the drive. You might also contact them and see if there are any other reports of this problem from Macintosh users who are using the drive to hold the startup volume.
If you open the system or console log using the Console application (located in /Applications/Utilities) do the attempts to boot that failed show up? I had hoped that a verbose boot would show that the system was waiting for the startup volume, but apparently you do not get any text on your screen when you boot after a shutdown and hold down the Command and V keys immediately after pressing the power button.
