You know you need to delete all your files when you get rid of an old computer. But just dragging all your documents folders in to the Recycle Bin and hitting Empty, logging out of Dropbox, clearing your browser history—that’s not enough. If you want to really make sure an opportunistic thief won’t find their way into your data, you need to clear the computer and set it back to the factory settings. (Yes, if you’re Jason Bourne and need to evade the data forensics team, then you’ll want to write over the disks. But that’s a little extreme for the rest of us.)
Fortunately, that process has gotten much, much easier in the last few years. As long as you have decent internet speed and a half-hour to spare, you can take a computer from lived-in to as-new with a few clicks.
Before anything else: get a cheap external hard drive, and make a copy of this computer’s data. Windows 10 has built-in backup features in its Recovery menu, while macOS has its snazzy Time Machine app. Use them in case you need to access anything you accidentally deleted.
Ready? Here’s how to do the rest.
1. Open iTunes. Go to Account -> Authorizations -> Deauthorize This Computer. You’ll have to enter your Apple ID and password to do this.
2. Go to Settings -> System Preferences, then click on iCloud. Hit the button to Sign Out. It’ll ask if you want to keep a copy of your iCloud data on the computer. You can click No, since you already have a backup, and iCloud automatically saves stuff like Contacts to an Apple server somewhere far away that you can access anytime with your Apple ID.
3. Open the Messages App. Click on the word “Messages” on the upper left, then Preferences, then Accounts. Highlight accounts, then hit the minus button on the bottom to remove them.
4. Get back to the desktop, or open a Finder window. Highlight Macintosh HD, and hit cmd-i. (On newer Macs, you can go to the Desktop, then click Go at the top, then Computer. Under “General,” there will be a field labeled “Format,” followed by either “MacOS Extended (Journaled)” or “APFS.” Note which one it says, you’ll need it later.
5. Close everything up, and shut down the computer. Turn it back on, and immediately press and hold Command and R. Release when you see the Apple logo or a spinning globe.
6. On the window of macOS Utilities, open Disk Utility. Then View -> Show All Devices.