How to prepare linux computer for donation?

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http://ubuntuforums.org – Hi All, I am interested in donating some of my computers. What do you suggest I do in preparation? In particular - What would be the best way to wipe the HDD contents? - Would it be best to donate without Linux on, or should I fresh install? - If it is better to install Linux before donating, what types of users should I create? Any convention? I was thinking "Administrator" or something as the sudoer. What do vendors do when selling a new computer with Linux on? Any suggestion is highly appreciated. Thank you very much! (Hardware)