A significant concern was brought up after we posted the article Clean Install vs. Deleting Users’ Accounts a few weeks ago. The question as to whether your deleted data can be recovered is something to be aware of, but it’s not necessarily something to worry about. It is a fact that data deleted from your hard drive can possibly be recovered by the appropriate recovery software. For most people, this is not something to worry about, as it takes time and effort to recover deleted data, and depending on how long the data has been removed from the hard drive for, it very well may not be recoverable.
Recovering deleted information is possible because of the process through which the hard drive stores and removes data. All information is stored in binary format: zeros and ones. A directory on the hard drive then points to specific groupings of the binary digits to make up files. When you use Disk Utility to erase a disk, you are emptying the directory. An Apple Support article uses a great analogy of “removing the table of contents from a book but leaving all the other pages intact.” Basically, your data is still present on the hard drive; however, the hard drive ignores the existence of this data, overwriting it as the computer continues to be used. Therefore, the longer the erased data stays stagnant on your hard drive, the less likely it is recoverable.
Disk Utility has a built-in “Secure Erase” option, which allows you to effectively erase a hard drive in such a way that the data almost certainly cannot be recovered. This feature will erase the hard drive normally (removing the directory), and then proceed to write zeros over the data. Every pass of zeros written over that data makes the recovery process that much harder. In reference to the aforementioned article written a few weeks back, Lion now has a “Secure Erase” option when removing user accounts. This performs the same function, but only to the removed user account/home folder.
It’s always a good idea to consider the option of securely erasing your data, depending on the situation. Whether you have extremely sensitive personal data, or are selling your computer to a complete stranger, it’s nice to have the option of extra security.
Note from Matt: The only way to be completely sure your data is unrecoverable is to physically smash the device into small pieces. Following the suggestions outlined here is fine for 99% of users, however.