The standard best practice is to delete a file is to move it to the "Trash" or "Recycle Bin", and then "Empty" that container. Until you empty the container, no space is freed up and you can still drag the file out and use it.
However, a computer's normal delete function does not erase the information from your computer. If you have some exceptional work related reason to make sure that a file you deleted cannot be recovered from your computer by file recovery software, you may optionally use these procedures.
These procedures are not the recommended normal way to delete files, as the extra trouble and time are ineffieicient for protecting ordinary confidential information. These procedures are only recommended if you inadvertently receive some restricted information (e.g. someone sends you their SSN in a resume) or you are passing the computer on to someone else and need to make sure they cannot access confidential data from deleted files. These procedures are not acceptable for protecting restricted information if you frequently receive it; contact the Information Security Office for whole disk encryption if you frequently handle restricted information on your computer.
The following instructions permanently remove the deleted data; can slow down your computer; and can take a long time. It may be best to do this over lunch or overnight on a locked computer. According to the relevant standards document from National Institute for Standards and Testing, a single "pass" or overwrite of a deleted file is sufficient to prevent its recovery by any file recovery software or machine. Therefore, there is nothing to be gained by selecting the more repetitive overwrite procedures available in deleted file removal programs.
- Move the files you want to permanently erase on your computer to the Trash.
- From the Finder menu, click Secure Empty Trash. You should see a dialog box that looks similar to the following:
- Click OK if you want to continue with the deletion(s).
To erase all free disk space, and thereby remove all traces of files that you deleted previously using the regular Empty Trash feature, you may use the Macintosh Disk Utility feature (located in the Applications folder):
- Open the Applications folder
- Click the Utilities Folder
- Double-click Disk Utility
- From the left-hand column, click the disk, volume, or image from which you want to erase free space
- Click the Erase tab
- Click the Erase Free Space button. You should see a dialog box that looks similar to the following:
- Click the radio button in front of the Zero Out Deleted Files option
- Click the Erase Free Space button
If you see a system message that says you are running out of disk space, you can ignore it. When the utility is complete, your hard drive will have the same amount of free disk space available as before you ran the utility.
To completely clear deleted data from the C: drive on a Windows computer:
- Quit all programs
- Click the Start button
- Click Run...
- Type cmd into the Open: box, and press the OK button
- Type cipher /w:C:\, and press the ENTER key
This permanently removes the deleted data which can take a long time if you are overwriting a large amount of data.
[updated Jan 2012 ISO]