How to Secure Empty Trash and Erase Free Space from an Old Mac
If you store sensitive data on your Mac, particularly on a laptop, get in the habit of selecting Secure Empty Trash from the Finder menu whenever you want to empty the Trash. Otherwise, your data remains in unassigned areas of your hard drive until such time as the operating system needs that space to store something else. Emptying trash and erasing free space is even more important when you’ve giving away your old Mac.
Secure Empty Trash can take a while if you’re deleting large files. Even if you’re careful to empty your Trash all the time, it’s a good idea to write over all the free space on your hard drive every so often. Fortunately, Apple supplies a tool for erasing free space from your Mac:
Open Disk Utility.
It’s in the Utilities folder inside the Applications folder.
Select your hard drive and click/select the Erase Free Space button. Then choose one of the three options.
The default option, which overwrites all free disk space with 0s just once, can take an hour or more. The more secure 7-pass method can take overnight, and the 35-pass method for the paranoid can take a couple of days. You’ll probably want to use the 7-pass method, unless your friend is coming over in an hour to pick up the machine. At the very least, before handing off your Mac, do the 1-pass erase.