Lingering Data
Wondering if deleted data can be recovered is valid to be mindful of (but of course, you’re backing up, right?), though not necessarily something to worry about.
For most cases, data deleted from a hard drive can possibly be recovered by the appropriate recovery software, but it takes time and effort. However, depending on how long the data has been removed from the hard drive has a bearing on the success rate — it very well may not be recoverable at all.
Recovering deleted information is possible because of the process in 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 able to be recovered.
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 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, Mountain Lion and Lion have 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, but it’s situation-dependent. 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.