I'm always installing a new Linux distro, and I was wondering if doing this deletes ALL files on the hard drive, I mean everything, like even a computer forensics expert couldn't find.

Depends on how much of the HDD was over written. If you zerofilled the HDD before doing this one. There is a lot of variables behind all this to answer a general question like this one.

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Iorana Haraharaini

Ann is correct it all depends however what peeks my interest is the comment about computer forensics. obviously something to "hide" but if its in some legal case I wouldn't destroy evidence it will only go against the person in the end. Seen it many times

Well it's nothing I did wrong, I'm just making a point, not even computer experts could find any files that were previously installed. Thanks for answering

As a generalisation, without specific qualifications of exactly which procedure is used, that statement is false. Unfortunately many people have the same misunderstanding.