It may be in the interest of FB to show no trace, but it would require a fair number of people breaking a bunch of laws. As the article pointed out, they've already done discovery/forensic analysis on these drives.
Tampering with that would be a MAJOR crime by everyone involved.
Huh? They could just do another analysis to show email from that time.
Although proving the lack of something is a lot more difficult that proving the existence of something. So it doesn't always serve much for them to say "nope, can't find them".
However, I am sure they will be eager to analyse Ceglia's computers for evidence
Tampering with that would be a MAJOR crime by everyone involved.