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@lispi314 >In truth, not pre-writing it doesn't really change anything if one didn't make the mistake of using it without encryption before.
On a HDD, overwriting everything with 0's (random overwriting is very slow and a waste of time) will actually overwrite almost everything.

What can get you is bad blocks containing old data that can't be overwritten (although you can painfully manually check for bad blocks and check the contents and some HDD's can be forced to write things to bad blocks I believe).

This is assuming that the proprietary R/W software on the HDD isn't specifically designed to retain certain information no matter what (in that case, you're indeed gone unless you make sure to always encrypt and even then there are some (albeit limited) attacks that can be carried out against block-level encryption).

Overwriting all blocks on SSD's with random data technically should work (0's may cause the controller to just mark the virtual block as empty and not overwrite anything), although that tends to cause premature wear and SSD's do keep a lot of bad blocks around.

Running a quick TRIM erase command will cause the controller to mark all blocks as empty (I'm not sure if controllers typically pretend there's no blocks afterwards, or will happily provide the stale blocks and even if they do implement so called "secure TRIM", controllers can be coaxed to do anything really).

It's not that terrible if you accidentally used no encryption before, as no newer information is leaked (although certain metadata from access patterns might be exposed for a while) and the old data progressively will be overwritten as the drive fills up.

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