When you delete a file on your Mac (or any other operating system), you don’t actually delete the file. The data for each files still resides on your hard disk and given good recovery techniques you can get it back. When you empty the trash all you Mac is doing is removing the entry to where you the file is located. Since the block of data on your disk doesn’t have an entry file associated with it, your Mac will write over the space. This is why files you have stored on your Mac don’t get over written. Now if you want to ensure those files are gone for what ever reason, I am soon going to do a block level duplication of my drive and I only want the actual data, you can erase the free space by over writing it.
This is really simple, it does take a while because your Mac has to physically write to every single block and byte on your drive. When the process is done, any data that was stored in the deleted space will be gone and unrecoverable.
To complete this task, open Disk Utilities located in Applications > Utilities. Select the drive you want to erase the free space on. Select erase from the tab at the top and press the button Erase Free Space.
An option box will appear, giving you three options. One pass, seven pass or 35-pass. One pass will go over every single byte that doesn’t have a file stored on it and write a set of zeros. Seven and 35 will do the exact same thing, but either seven or 35 times respectively.
I recommend the one pass setting for pretty much everything but the security conscious. Seven pass is suitable if you have a lot of time, 35-pass if just ridiculous.
You mac will proceed to write to disk. Assuming your can write at an average of 30mb/s (check in Activity Monitor) 1Gb will take 34 secs. So assuming its 2Gb a minute you can quickly work out how long it will take.
During this time Disk Utility will become a bit unresponsive. Just leave it and let it run. It will tell you when it is finished. Note that the process will begin to eat up disk space. Eventually you will have no space left on your main drive. After your disk is full, the app will then delete this file and the process will be complete. Note that when you have no disk space left on your Mac, doing general normal things becomes a bit difficult.
After it has finished your disk will be clean and no other data will be stored. If you want to do this every time you delete a file use the secure empty trash option in Finder.
If you have any questions or comments on this tip, please leave one.