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Securely erase disks with Disk Utility (10.4/5)

Safari URL listWhen files on a disk drive are deleted you might think that they are completely disposed of and that no-one can recover them. When a file or folder is sent to the Trash and you then empty the Trash folder, then that's it isn't it? The files and folders are gone forever aren't they?

No, this is not true and someone armed with the right tools is able to recover the files you deleted, even when you have emptied the Trash folder. The reason is that when a file is deleted by the operating system, it does not wipe the part of the disk containing the file. OS X keeps track of all the files on the disk and it simply marks the space occupied by the file as unused and free to be used again to store another file. The problem is that disks have a lot of free space and the small part that was occupied by the file might not be used again for quite some time. Until it gets used to store some other data, the original data is still there. A technical person armed with the right software tools is able to find that deleted file and to recover the data. This is clearly a security problem.

If you sell your Mac or throw it away you should ensure that the files and information stored on the hard disk drive cannot be recovered and really is deleted. The same is true of an external USB or FireWire hard disk drive or a USB flash memory drive. Wipe it or the next person to get their hands on it might recover the data and have access to all sorts of personal information. They could access your online banks or favourite stores, spend your money and even steal your whole identity. It's clearly an undesirable situation, so what can be done about the problem?

Fortunately, Disk Utility has some useful functions for cleaning up disk drives and for preventing data on them from ever being recovered. Go to The Applications/Utilities folder and double click Disk Utility to start it. Select one of the disks in the left panel and click the Erase tab.

If you only have one disk drive - the startup disk - the only button that is enabled is Erase Free Space. This is useful for ensuring that any files that you previously deleted on the disk can never be recovered. Click it - nothing bad will happen. You are presented with three options:

  • Zero Out Deleted Files: This is fast and experts that work for data recover firms say that with the high bit densities of modern hard disk drives, overwriting a file just once means that more often than not, it is impossible to recover.
  • 7-Pass Erase of Deleted Files: This overwites previously deleted files seven times using different bit patterns. This is more than sufficient for modern disk drives. You might hear stories about overwritten files being recovered, but that was 10 years ago when disk drive mechanisms were really loose and inaccurate.
  • 35-Pass Erase of Deleted Files: Ask any data recovery company if they can recover a file overwritten seven times and they'll say that can't, so why bother overwriting them 35 times? This takes a very very long time and and is for highly paranoid people who think everyone is out to get them.

If you select an option and click the Erase Free Space button Disk Utility will begin the task. It can take some time even with the quick Zero... option, so go and have a cup of coffee and come back later.

If you have a external FireWire or USB disk drive, or a USB flash memory drive, you can use the Erase button to wipe it clean. Before you click Erase though, click the Security Options button. You'll see the three security options mentioned above, plus one more option - the default - Don't Erase Data. select one of the three secure deletion methods and click OK. Now you can click Erase to completely wipe the disk drive.

What if you want to erase the Mac's internal disk drive before throwing it away (actually, you should take it to an approved recycling point or pass it on to someone else so they can get some more use out of it). Disk Utility won't ket you erase the startup disk. If you have used a program like SuperDuper or Carbon Copy Cloner to back up your Mac to another disk, you can boot up with that disk and run Disk Utility from it. You can then select the Mac's internal disk drive and erase it. Another option is to start the Mac using the OS X installation DVD. You can then use its tools to erase the internal disk drive.

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