It may be wise when selling or donating an old Mac to wipe all of your data off of it and offer the system to the new owners as close to how it came out of the factory. This not only offers them a fresh start, but also ensures your data is kept safe and secure.
One common practice I see people do regularly when selling their old systems is simply create a new account for the new owner, and then hand the system over while it still contains their old account, installed applications, and other details. While this might seem like a quick way to set up your Mac for a new owner, the best approach is to fully format the system and reinstall OS X; however, there are a few additional steps you might need to take.
The first step in any factory-reset of the system is to completely back it up. You can do this using Apple’s Time Machine service, or with a system cloning tool. Either method should result in a full copy of your system that you can restore or migrate to a new system, if needed. A backup will not only help secure your data, but also be another option for transferring your data and settings to your new Mac.
2. Disable online accounts
One of the more important things you can do before formatting your hard drive is to disable any online accounts you use with the system. Even though most can easily be transferred to new systems, some like Apple’s iCloud may be linked to your Mac’s hardware and result in you having to take additional steps to remove the system from your account. Having your hardware linked to accounts may also prevent the new owner from using the system with these same services, so take a little time to disable them before selling your system.
3. Remove any third-party hardware
If you have added any upgrades or expansions to the system, then you might consider removing them. Granted RAM and hard drive upgrades may be difficult or unnecessary to undo since they are specific for your system and your new replacement Mac might be far more capable and not need (or even be able to use) any components from your old mac. However, if you have a Mac Pro with special PCI-express cards installed, then unless you are selling them along with the rest as a package, consider at least removing them from the system.
4. Reset hardware
Your Mac contains a hardware settings that include iCloud information among other customizations, and while these will be reset by a new user, be sure they are in a state as they would have come from Apple. If you have set up a firmware password on your Mac, then be sure to disable this before handing your Mac to a new owner, as it will prevent the owner from performing any maintenance. After ensuring this password is disabled, then reset your Mac’s system management controller, and reset the PRAM on your Mac to give them default values.
5. Format and reinstall
The next step is to format your system and resintall the original operating system software that came with it. While you can install the latest version of OS X that you purchased from the App Store, technically this is a copy that is licensed to you, and not to the new owners. Reinstalling the original version of OS X will allow the new owners to choose what version of OS X they would like to upgrade to.
To reinstall the original version of OS X, if it came with a gray installation and restoration DVD then insert this into the optical drive and reboot with the C key held down. Then after selecting your language choose Disk Utility from the Utilities menu. In Disk Utility, select your internal hard drive device (listed above any volume names contained on it), and you should see a “Partition” tab appear. In this tab, choose “1 Partition” from the drop-down menu, and then give the partition a name, set it to be “Mac OS Extended (Journaled),” and apply the changes.
If your system did not come with a gray restoration DVD, then you will need to use Internet Recovery to restore the original version of OS X that came with your system. This will access Apple’s servers and download the installation tools for the version of OS X that came with your system. To do this, ensure you have an appropriate internet connection for use with Internet Recovery, and then reboot with the Option-Command-R keys held to force loading to Internet Recovery. Again, use Disk Utility to fully partition the internal boot drive, and then quit Disk Utility.
Once the hard drive has been formatted, you can proceed with running the OS X installer, and wait for the content to download and be set up on the system.
After the installer completes, the system will restart and show the welcome screen of the OS X setup assistant, at which point you can hold the power button to shut off the system (when it is next turned on it will re-start the assistant). It is now ready for the new owners to turn on and set up, as if the Mac arrived just out of the factory.
This is the best way to sanitize a disk:
2. Fill with a particular large file, like a movie.
3. Fill with a smaller file.
4. So on, intil 0 kb are left.
5. Format again.
That way you can be sure no previous files are left, because all has been rewritten. All means all.
Is there an application, script o similar way to automatically copy a particular (large file) into a disk many times as needed, until the disk is filled?
Use Disk Utility to write zeros. Easier.
Nope. That does not fill the disk. Some uncovered space may remain. But filling it with real files does erase all previous stuff. That is the difference.
under point 2, add in to use iTunes to “de-authorise this computer”