Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
How to erase the data on your Mac completely
For most of the people it is not a secret that when you delete a file on a Mac(or any other computer)you don’t actually erase it completely. The data for every file stays on your hard disk and, using a good recovery technique, it’s possible to restore it. Basically, all your Mac is doing, when you’re moving files to Trash is removing the entry to where your file is located. Consequently, the block of data doesn’t have an entry file, associated with it, but that data is still there – that is why, Mac write the new information over the free space on a hard drive. This is why the files you stored on your Mac don’t get erased completely, when you “Trash” them.
Is there a way to get rid of those files completely, you might ask? Yes, there is a very simple way to erase deleted files by overwriting them. It takes a while, because what your Mac does is it physically writes over every single block and byte on your hard drive. Once the process is done, any data that was stored in the free space will be gone and unrecoverable.
In order to complete this task you need to open Disk Utilities, located in Applications > Utilities. Choose the drive you want to erase the free space on. Next, select “Erase” from the tab at the top and click the Erase Free Space button.
You’ll be given three options in an option box that will appear – “zero out deleted files”, ”7-pass” or “35-pass”. One pass will write a set of zeros over every single byte that doesn’t have a file stored on it. Seven and 35 pass will do the exact same procedure, but either seven or 35 times.
One-pass setting is usually good-enough for pretty much everything. Pick 7-pass if you’re concerned about the security of your very personal data, though, keep in mind that it’d take a while. The last option takes 35 times longer than “Zero Out” and rewrites over the unused disk space 35 times.
Your Mac will start rewriting the disk. Assuming the average speed your Mac can write at is about 30 MB/sec (check in Activity Monitor), 1 GB will take 34 seconds. Now you can do the Math and figure out how long it’d take to complete this operation.
The Disk Utility will become a bit unresponsive during this process. So just leave it and let it run, it’ll tell you when its done. This process will eat up your disk space. At some point you’ll run out of space on your main drive. After your disk is full, the application will delete this file and the process will be completed. However, you should remember that the lack of free space on your Mac will make doing average things a bit difficult.
When the process is over, your disk will be wiped out of all unwanted data. If you want to do this every time you delete files, use the secure empty trash option in Finder.
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