Re: Giant degausser
Modern drive heads don't actually read absolute magnetization. Instead, they read relative magnetization from one sector to the next. Those sectors are very, very close together so any macroscopic magnet is going to hit all of the neighboring sectors almost as hard as the target sector, meaning no change in relative magnetization. Once you hit saturation, you can start breaking data, but experiment shows that the fields needed to get to that point are sufficient to physically rip the platter apart. Your best bet by far is the sector-local fields you can generate with the write head of an operating drive, even compared to physical destruction.
This is not to say that magnets are harmless to hard drives - they can cause head crashes in a running drive. But if you're worried about NSA-level data recovery efforts, a giant degausser will do nearly nothing to corrupt the data.