When you put a file into Trash and then empty the Trash, you aren’t really deleting or destroying that file. If you really want to destroy that file, you should use Secure Empty Trash, which will overwrite the file. To do this, put a file into Trash, then click on the Finder menu and select Secure Empty Trash. 

[Update: December 2015. Starting with OS X El Capitan (aka OS X 10.11), Apple has removed the Secure Empty Trash option because it can’t be guaranteed to work with solid-state drives (SSDs). This Intego article discusses the situation and the ways to securely erase files in OS X El Capitan.]

Typically, when you select Empty Trash the file is not destroyed. Instead, the only thing that happens is the removal of a record indicating that the file exists in some assigned location. The removal of this record indicates that the file’s storage location is available for re-use. Some day, maybe many months later, the computer will store a new file in that same location. This storage process over-writes the original file. This effectively destroys the original file. As you can see, files are never really deleted. File deletion is really a misnomer. Instead, the way to delete a file is to over-write it.

The Secure Empty Trash command does just this. Choosing Secure Empty Trash causes files to be over-written. In fact they are over-written seven times! Be aware that over-writing a file seven times takes time, so securely emptying your Trash takes noticeably longer than merely emptying your Trash. If you’re deleting hundreds or thousands of files, don’t be surprised if it takes 30 or 60 minutes or longer to securely empty the Trash. If you would like to configure your Macintosh to automatically use Secure Empty Trash, by default, as opposed to the old-fashioned Empty Trash, do this:

• Go to the Finder menu, select Preferences.
• Click the Advanced button.
• Check the box labeled Empty Trash securely.

If you’re going to be getting rid of your computer and want to securely delete (over-write) the entire hard drive, check out a previous Tech Tip about How to Securely Erase a Mac’s Hard Drive.

Note: When some people learn that emptying the Trash doesn’t really delete or destroy files, they get lured into a false sense of security of thinking that these non-destroyed files can be undeleted or recovered. Technically, it might be possible to recover such files, but it is a painstaking process. Here’s a quick overview. First one needs to use a file recovery application to scan the entire hard drive. This scan typically takes hours. During the scan all files which have been deleted, but not yet over-written, will be located and copied onto some other disk. This typically produces folders full of hundreds or thousands of files. It’s important to know that file names are not recovered. Thus, recovered jpeg photos might be named something like image1.jpg, image2.jpg, etc. Thus, after spending hours scanning for and recovering files, one has to spend more time examining each file trying to locate the desired file. As you can see this process is very time-consuming, so don’t rely on it. Instead, set up a backup system using Time Machine. Then learn to restore files from a Time Machine backup. This process typically takes only a few minutes.