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 Securely Erase Your Mac’s Hard Drive
Unlike many Windows machines, Apple computers hold their resale value for several years after being purchased. This makes them a valuable commodity for computer users who are thinking about upgrading to a new machine, as selling the old could result in a hefty payday that will help ease the cost of a new computer. Whenever selling an old computer, however, it is important to make certain that any of your sensitive data has been removed from the machine, in order to protect your privacy. Fortunately, it is relatively easy to entirely erase your Mac's hard drive.
Erasing your hard drive is not as simple as reinstalling the operating system. While many computer users choose to rewrite their data in this way, this only hides information on the surface level. An advanced computer technician will be able to dig deeper into the machine, pulling out any files that may have been left behind in your erasing. If your old computer were to fall into the wrong hands, this could potentially have devastating consequences leading to issues like identity theft.
Before beginning, you will want to back up your data to an external drive. There are several different ways to go about doing this, though Time Machine and Disk Utility are arguably the most popular options available. Once you've backed up your machine, be certain to verify that the backup is functioning and all of your files are accessible.
Armed with the Mac OS DVD that came with your system, load the disk and hold down the option key. Your machine should give you the choice to boot from the machine's drive or from the DVD. Choose the DVD, and instead of installing the operating system, click the Utilities menu at the top of the screen. From there you will be able to launch Disk Utility. You'll want to click your drive, and then choose to erase it, selecting Mac OS Extended (Journaled) as the format.
Beyond those options, you will see the Security Options prompt at the bottom of the dialogue. Click here to see all of the available erasing techniques available to you. Simply select the option that you feel is most appropriate to your situation, and then erase your machine. You will then be able to install the operating system once again with the Mac Install DVD.
When selecting the security that you wish to utilize for formatting the machine, you may be tempted to opt for one of the single pass rewrite options. While this does erase some data, it is not as thorough as one of the 7 or 35 pass options. Of course, opting for a rewrite that takes place multiple times will take much longer, but it is also more thorough and will keep your machine safer. If you don't want to spend unnecessary amount of time rewriting data on your computer, you might instead opt for one of the lower security options, and simply perform the erase multiple times. This will save you time, but also protect your machine further than one single erase.