How fast will it take for eraser to remove 16 gigs of files if lets say the cops stood at the door? Then only using remove 2first and 2 last kb and does it do it induvidully or does it take the whole bunch?
So definitely don't make a run for your computer when the door gets busted down - it'll probably be the last thing you ever do.
Also, it would require a massive amount of sangfroid to be able to accomplish the partial erasure of the files under that kind of stress - and would it really do any good? All the rest of the information would still be on your HD, and available for recovery (albeit they'd have to work a lot harder for it).
Given the fact that you're probably going to be sound asleep when the door comes off the hinges, I really think this is all pretty much a moot point.
The only way to not have to worry about stuff like this is to not have anything on your computer that'd ever draw that kind of attention from the authorities - period. (Or to have such material totally encrypted, which is a total PITA). Pete