How do u completely erase data from Mac before selling?

For normal files, just use the delete function and clean the trash.

For important files, open the files and delete all the content; then delete and clean trash.

Make sure back all the important files before making such decision.

Secure Erase can be used. Also, unwanted accounts should be removed via System Preferences User Accounts. That will get rid of everything if you don't select the copy to a file option. Just be sure that if an Admin account is being deleted, that you create a new one first. The system will require this be done.

Before you sell or give away your computer, in addition to the steps listed in this support article, take these steps:

1. Run Apple Diagnostics or the Apple Hardware Test. The buyer will do this (or he should), and you don't want to be surprised by the results.

2. Reset the PRAM and the SMC, which might contain personal information.

3. If you set a firmware password, remove it by running Firmware Password Utility in Recovery mode.

4. If you activated FileVault in OS X 10.7 or later, turn it off.

5. If you use Boot Camp, the partition must be deleted.

6. If you created any other data partitions on the internal drive, remove them in Disk Utility.

7. If the machine has an internal hard drive, erase the one remaining data partition with the option to zero out data. An SSD doesn't need to be zeroed.

8. Remove the machine from your list of registered products. If it's still covered by an AppleCare Protection Plan, transfer the coverage to the new owner by following the instructions in the AppleCare Terms and Conditions (under the heading "Transfer of Plan.")

Also note:

You can't legally or practically transfer any software downloaded from the Mac App Store to the new owner of the machine, even if it was free. That includes OS X, so if you upgraded to OS X 10.7 or later, you must reinstall an older OS, either from the installation media, if applicable, or by starting up in Internet Recovery mode (option-command-R at the startup chime.) If you installed from physical media, deliver those to the new owner.

Replacements for the original media can be ordered from Apple.

If the machine shipped with OS X 10.4 or 10.5, then you may be able to install on it from a retail Snow Leopard disc (which you must then transfer to the buyer.) To run Snow Leopard, the machine needs at least 1 GB of memory. The buyer should understand that if he doesn't get the original media from you, he won't get the bundled iLife applications or the Apple Hardware Test.

The new owner will have to redownload any software that came from the App Store, if applicable, including OS X upgrades, under his or her Apple ID. If you ever updated the bundled iLife applications (Garage Band, iMovie, and iPhoto) through the App Store, you can't transfer those either. The buyer will have to purchase them. Note that iPhoto has been discontinued and is no longer available for purchase.

Geraldpau wrote:

For normal files, just use the delete function and clean the trash.

For important files, open the files and delete all the content; then delete and clean trash.

Make sure back all the important files before making such decision.

Don't you know that simply delete a file can be easily recovered by some recovery app, I need my data never to be recovered, any suggestion?

If you prefer to delete the files permanently, then the entire disk should be written a few times with a random set of digits. You can do it via command line but it requires programming tactics to be involve. However, there are simple solutions out there for erasing a Mac, like this: How to Wiper a Mac before selling.

Make sure the files were backed up before erasing your mac.

Save all of YOUR data to a USB stick or external HD prior to doing the following:

Power up holding [command]+[R] to enter Recovery. Open Disk Utility and click to highlight "Macintosh HD" click Erase tab. Erase Macintosh HD. Rename it to Macintosh HD if it shows as "Untitled". exit Disk Utility. Click "Install New Copy of OS X". After setup completes and you see the WELCOME screen, shutdown. It will then be ready for sale or gift.

If you want a more secure erase. In the Erase tab of Disk Utility, click Security Options and move slider to more secure. First tick mark takes 1 hour and 15 minutes for one pass of zeros on a 500 GB HD.

Erasing in Recovery is secure. Link Davis also provided valuable information as well. If you use max setting in Security Options, It will make a D.O.D. comparable secure erase. Takes hours to complete.

Cheers!!