Can’t erase your Mac’s startup disk? Try Internet Recovery as a last resort

When you’re trying to wipe an old Mac to set it up fresh or to give it away or sell it, you typically want to erase the drive. Sometimes, you’re thwarted.

The easiest sequence is the following:

1. Select  > Restart if your Mac is running.
2. Just as the computer starts, hold down Command-R, which loads macOS Recovery.
3. Wait until the startup progress bar appears and release the keys. (If you see a globe, skip down to the end of the article.)
4. When Recovery starts up, select Disk Utility and click Continue.
5. Make sure you’re selecting your macOS startup partition and not the entire drive, which contains the macOS Recovery partition you’re currently using. Fom the View menu, select Show All Devices.
6. Select just the partition or container with macOS and click the Erase button.
7. Quit Disk Utility and use Reinstall macOS from the main recovery window.

It’s possible you’ll hit an error at any one of these points.

For instance, some readers report receiving the message: “Secure Disk Erase Failed—Couldn’t Unmount Disk.” This happens in a few circumstances, but most of the time because you’re trying to erase the entire disk—consult steps 5 and 6 above.

If that doesn’t help, because you’re trying to erase the entire drive, you can shift to internet-based macOS Recovery, which downloads the recovery software temporarily from the internet and can erase the entire drive. For most Macs in recent years, restart and hold down Command-Option-R. (For Macs running a version of the operating system before Sierra, the key combinations will be different.)

In a few moments, you should see a rotating globe, which indicates your Mac is pulling down the necessary files over the Internet. You’ll be prompted at some point for the name and network password for a Wi-Fi network, unless you’re connected via Ethernet to a network.

You should now be able to proceed to erase the disk in full and reinstall a fresh copy of macOS.

This Mac 911 article is in response to a question submitted by Macworld reader Dan.

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