I'm not familiar with sony so if there are any known issues with those laptops you have, I don't know them.
Usually when a laptop does not see a hard drive the laptop trys to boot from everything else including the network. It usually doesn't say operating not found unless it sees the hard drive, but the operating system is not present or corrupted. Download Hdat2 burn it to a cd and boot from it, test your hard drive and see if the Smart attributes are good then try restoring the hard drives.
It could be that some of your hard drives went bad, its not uncommon, especially with laptop hard drives.