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Do it Mr. Robot style, drill through the top of the HD then stick your RAM chips in the microwave!
I don't usually go that far.
For a standard mechanical drive, a single pass zero fill is usually sufficient.
For SSD (including mechanical drives with onboard SSD cache) it gets a whole lot more complicated, because what your computer sees versus what is stored in the flash memory can be quite different:
- Controller chips use fancy wear levelling algorithms that spread writes evenly over the flash, so two writes to sector #1234 could be stored at two different points within the flash memory.
- A zero fill is not sufficient because the controller chip will be smart enough to compress that, and it will leave the large majority of the sectors intact internally.
- The SATA standard has a special "secure erase" command, which is supposed to clear absolutely everything. Some SSD firmware messes this up, not erasing everything, so it appears to be a zero filled drive even though the data is intact.
- Truly random data cannot be compressed (since it's random) so an overwrite with random data should clear every sector. But for mech+SSD drives there's no guarantee that data will ever be written to the cache SSD, so there could still be remnants of the original data sitting in that flash memory.
And what happens if you don't have physical access to the drives? ALWAYS do a random data fill when you leave a web host...
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