I know anyone want to make some money from an old computer, sometime gettting less money isn't the worst thing. Your HDD or SSD preserve a lot of your important data like your family trip images or all your work files. When you delete a file, the file is showed how "deleted" but instead is still on the disk. The file will be still on the disk until you don't overwrite it with another file, for that reason there are some tools that are made to overwrite and make impossible to recover a deleted fi