How to Erase Free Space on an iMac (5 Steps)

Quit all running programs and empty the Trash. To empty the Trash, right click on the 'Trash' icon in the Dock and select 'Empty.'
Click on the 'Applications' folder in the Dock or open a Finder window and click 'Applications' in the left pane under Places. Double click the 'Utilities' folder to access the contents of the folder. Double click the 'Disk Utility' icon to launch the application.
Select the iMac's internal hard drive by clicking on the 'Macintosh HD' icon in the left window. Click on the 'Erase' tab and click 'Erase Free Space.'
Choose to have zeros written over the free space once--seven times or 35 times--by clicking the radio button next to the option. Depending on the amount of free space on the hard drive, choosing the '7-pass' or '35-pass' option may take many hours to complete. Click 'Erase Free Space' to start the process.
Quit Disk Utility once the erase process is complete by hitting 'Command' and 'Q' on your keyboard or by choosing 'Quit' from the Disk Utility file menu.
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