But wait, there's more! Next time you want to clean something, hold down the Option key on your keyboard before clicking Clean…. You'll see that this button changes to Secure Clean…:
Secure cleaning does more than just delete the selected item(s): it overwrites the item(s) with random data in the place where it (or they) used to be on your hard drive. This is useful in situations where you think someone might use a special data-recovery program to undelete the file you've cleaned.
Such programs work by taking advantage of the fact that computers don't usually truly delete files: rather, they throw away information about where those files are stored, in essence forgetting that they existed. Data-recovery programs are like private investigators who root through trash cans and piece together bits they find until the entire file has been reconstructed. They then copy the file, so the computer now sees it as it was.
Washing Machine offers three levels of secure cleaning:
- Basic writes random data over your files once before deleting them. This is the default setting.
- Medium writes random data over your files seven times before deleting them.
- High writes random data 35 times.
As you can imagine, secure cleaning takes much longer than regular cleaning. The time this takes depends more on the number of files involved than their size: while Bookmarks, Cookies, Browsing Histories and Download Histories tend to be stored as single files, Caches, for some programs, are usually large collections of files, and therefore take much longer to clean securely. Also, the higher the level of security, the longer it takes; medium security takes seven times as long as basic, and high security takes about 35 times as long. However, these higher security levels ensure that no one will be able to recover your data. When you launch a secure cleaning task, a progress bar appears while you're waiting.
You can change the security level by choosing Washing Machine > Preferences… and clicking the General icon.