When you delete a file from your computer, it doesn't really erase the file, all that you do is remove references to that file. Anyone with a third-party recovery program can potentially extract that data that you thought you've erased.
So how do you keep people from accessing your deleted files? The only thing you can do is to write over the files--this copies non-sensitive data over your old files, making them unrecoverable. Usually this is done by writing zeros over your data; the more times it writes over the data, the more secure the removal is.
In most operating systems, the only way to do this is to reformat the drive; but this removes all the data from the drive, including data and programs you want to hold onto. This becomes a labor-intensive process.
Luckily, the Macintosh OS X operating system includes a feature that allows you to securely delete your files without having to reformat. The Finder includes a Secure Empty Trash feature that not only securely erases data, but its seven pass method meets the U.S. Department of Defense standards for erasing files.
To use this feature, move the files you want to securely erase to the trash, either by dragging and dropping, by selecting the files and choosing File > Move to Trash, or by selecting the items and pressing Command-Delete on your keyboard.
Once the files are in your trash, Choose Finder > Secure Empty Trash from the menu bar.
You'll receive a verification dialog that warns you that you're about to securely erase items in the Trash forever. Click OK to continue.
Depending on the size and number of files that you have chosen to delete, this process can take a while to complete. Finder will show a progress bar that will allow you to monitor its progress, though it won't tell you how long it might take.
Later, we'll go over how to securely erase your free space on your Macintosh, including previously deleted files, by using Disk Utility.
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