When you delete files in OS X, by default the system will simply remove the file’s entry from the drive’s index, which simply allocates the space used by the file as free for other uses. However, during this logically deletes the file, on conventional hard drives the data structure of the file is still intact and with special file-recovery software can be read through and potentially recovered.
This means that if you store an important document on an external hard drive and then decide to delete the file, its contents will still be on the drive and may be recoverable. However, Apple does offer a few approaches for removing the residual data from a file that you are going to delete, or which has already been deleted.
Note: The following methods will PERMANENTLY delete the files in your trash and other files you have previously deleted. You will NOT be able to recover them afterwards.
If you have sensitive data on your machine and need to delete it securely, there are two methods to use:
Secure Empty Trash
If your file is still available, then the first option is to secure-delete it using the Secure Empty Trash option in the Finder. This will perform a multipass overwrite of the file’s contents with random data before the system finally removes the file. Keep in mind that this process will take far longer to do than a basic deletion, and its time frame will depend on the file size.
For instance, if you secure-delete a small text document, it will be done in split seconds, whereas, if you do so for a 2GB movie file, it will take number of seconds to completely erase the file.
- Move the files into your trash (at the end of the Dock).
- From the Finder menu, select Secure Empty Trash. This may take some time, depending on how large the files are.
Erase Free Space
The security empty trash option will only work if you have existing files, it you cannot be used on an already deleted file. However, Apple does have a service that can be used to ensure that deleted files are not recoverable, which is to secure erase the entire free space on a drive.
- Launch the Disk Utility Application, from Applications/Utilities on your hard drive.
- Select the name of your hard drive (e.g. Macintosh HD) in the left-hand pane of the Disk Utility window.
- In the right-hand pane, choose Erase Free Space. You may choose from Zero out Deleted Files,3-Pass Erase, 7-Pass Erase or 35-Pass Erase of Deleted Files.
The Zero Out Data is the quickest free space erase option, which provides good security over the unused disk space (one pass), taking the least amount of time. Because the 3- Pass Erase or 7-Pass Erase writes data over the free disk space seven times, which ultimately provides a highly secure erasure of disk data. A 3-Pass Erase takes three times longer than a Zero Out Data erase, as 7-Pass Erase will be 7 times stronger. And if you want to secure your data and not want to take even a little bit risk, then you can go with 35-Pass Erase. Though, the US Department of Defense recommends that 7-Pass Erase would be enough, as 35-Pass Erase is an overkill.
The more erase pass you will choose, the more time it will take to secure the data and overwrite it. As even with a “Zero Out Data” option, we saw the initial remaining time of 30 minutes raises to more than 1 hour. While it also depends on the size hard drive as well as the number of files in it, but the point here is that you would have to be patience if you want to secure your files from never been able to retrieve by some stranger. So, it takes at least an hour for 1TB Hard Disk to secure data with Zero Out option, if you select 7 or 35 Pass Erase, it will take at least 7 & 35 times more for these options to secure your data. Now the choice is yours.
Note: With an SSD drive, Secure Erase and Erasing Free Space are not available in Disk Utility. These options are not needed for an SSD drive because a standard erase it difficult to recover data from an SSD. For more security, consider turning on FileVault encryption when you start using your SSD drive.