We have a programmer here that used to work at TRW and he says they can actually go back and recover data that has been written-over or deleted more then 7 times.
Pretty high tech stuff.
If you have that much of a secret, stay with smoke-signals and off your computer.
This method involves using the edges of the tracks.
The more data they put onto hard drives, the less well it works. They can only recover some of the bits, not all of them. So your text data might or might not be readable. Binaries would be hopeless.
In any case, if the NSA is spending thousands to analyze your hard drive, you're in big trouble already.
Yes, because no one can see your smoke signals.
What they don't tell you is that doing so is extremely time consuming and expensive, and results in a jumble of isolated files or even just blocks that then must be pieced back together. Very expensive to do and very unlikely unless there is an extremely good reason someone wants to recover that data.
There are disk washing programs that will overwrite every byte of data on the disk effectively rendering it wiped without the use of expensive laboratory-type procedures.
Drive platters are being made of glass. A nail through the platter ought to do it.