A number of Seagate hard drives from the following families may become inaccessible when the host system is powered on:
Barracuda 7200.11
DiamondMax 22
Barracuda ES.2 SATA
SV35
Once a drive has become affected the data becomes inaccessible to users but the data is not deleted. Seagate has isolated this issue to a firmware bug affecting drives from these families manufactured in December 2008.
Seagate has a number of instructions to correct the issue and to see if your hard drive is affected by the problem.