Hmm.
Note: Not an expert at all nor have i really looked into this, but according to this post dating back from 2014 (so things might very well have changed since then), it might indeed be smart to manually overwrite your HDD after deleting.
https://bitcointalk.org/index.php?topic=425465.0We have shied away from tackling this problem, because it's really out of scope for us (the core devs have too). There's so many different filesystem types that have their own ways of handling data on disk. We could do a PhD on how secure-delete files on each one, and it still would probably not be perfect, and only handle a subset of the filesystems out there.
My recommendation is that if you really want to wipe it, you shred the entire drive from a live CD and then reinstall your OS. Alternatively, one thing that theoretically should work, would be to make sure the wallet is deleted, and then do something like "dd if=/dev/urandom of=tempfile.bin" (if linux), which will create a single file full of random bits that will eventually consume all unused space on your disk. This will force the OS to overwrite all sectors marked as deleted. Then, if the system is still functional, you can remove the file. Repeat a few times to enforce "shredding". Though, depending on the OS, strange things can happen when you literally fill your last byte of free space.
Again, not sure how accurate this still is.
