Here’s how to boot your M1 Mac from an external hard drive


It’s not as easy as it used to be as it will likely require you to buy new hardware. […]

You may want to use a bootable external drive to have an SSD with a higher capacity than what Apple is offering or affordable via pricing (c) IDG

Apple’s relatively new M1 Macs, based on Apple Silicon, have a number of usability differences from previous Intel-based Macs. One difference that has unsettled many readers is whether the M1 Mac can be started or booted from an external drive. Intel Macs generally make this easy.

You may want to use a bootable external drive to have an SSD with a higher capacity than what Apple is offering or affordable via pricing. Or you might want to have one as a backup in case something goes wrong with your M1 Mac.

Tests have shown that in order to boot from an external drive:

  • A Thunderbolt 3 drive. It’s not just one that uses the USB-C port, it’s a native USB 3.1 or 3.2 drive. You also cannot use a Type-A adapter for a drive with USB 3.0 or higher. Success seems to require a native Thunderbolt 3 drive.
  • Completely erasing the drive and then formatting it as APFS.
  • Obtaining a Big Sur installer and then installing Big Sur from your M1 Mac directly on the external drive.

Let us explain each point in more detail.

Thunderbolt 3 drive

Most inexpensive external drives use a variant of USB 3 to connect via USB-C. Thunderbolt 3 is generally reserved for high-performance drives and drive arrays for graphics and video purposes. However, One World Computing offers a special range of cheaper, bus-powered Thunderbolt 3 SSDs. (Some people have apparently been able to get a USB 3 drive to work with it, but no one has narrowed down which or why, so it’s impossible to recommend it as a course of action).

With an SSD inside, OWC charges 195.10 euros for 480GB and 299.87 euros for 1TB. You can purchase higher capacities or just buy the Envoy Express enclosure for $ 95.31, which you can install any SSD that is designed for the 2280 M.2 NVMe standard. (That sounds like a long sentence, but you can search for it to find compatible SSDs.) OWC says it supports current capacities up to 4TB and is designed to support higher capacities in the future. For the time being, I decided on a relatively cheap 500GB SSD (around 75 euros) in order to have a bootable option.

Erase and format as APFS

To use Big Sur, the drive must be formatted as APFS. However, it has been reported that it is not possible to simply change the formatting of an existing drive as invisible partitions used for purposes related to booting an Intel drive from a previous macOS installation on the drive could cause problems. To avoid that, select the drive in Disk Utility, click Erase, and follow the instructions to create a single APFS container. This should wipe out any conflicting data structures.

Obtain the Big Sur installer

Since you need to be running Big Sur on an M1 Mac, you should be able to download the installer directly from the Mac App Store using this link. Big Sur 11.1 or higher is required.

Install Big Sur on the external drive

Launch the Big Sur installer and select the external drive as the destination. Follow the prompts and steps. When your Mac restarts, it will boot from the external drive to complete the installation.

Reboot from your internal drive or switch between

To revert to your internal drive as a startup disk, you can open the Startup Disk settings window while macOS is running on the external drive and select the internal drive. © The n click on restart.

You’ll need to disconnect the external drive after the reboot and some people have reported Big Sur reporting that one of its partitions is still in use. (Catalina and Big Sur invisibly divide a macOS into a partition with system files and a partition with your user data; the data partition may not be separated correctly). You might prefer to shut down, unplug the external drive, and restart at this point.

You can also use recovery mode to change the boot drive. This is a little more complicated on an M1 Mac than on an Intel Mac, where you can simply hold the Option key and select a drive when you reboot (unless you have certain security settings enabled, in which case you’ll need to use Recovery Mode to disable it).

Here’s how to change the startup drive from recovery mode using an M1 Mac:

  1. If macOS is running, you need to shut it down. A restart does not work. Select> Shut Down.
  2. When you see your Mac shut down, press and hold the power key until you see a prompt that says, “Loading startup options.”
  3. When the “Options” icon appears, you’ll see a list of partitions next to it from which to choose. Select the partition you want to boot from.
  4. Click “Next” and the Mac will restart from this disk.
© The  recovery mode home screen on an M1 Mac lets you choose an alternate boot drive (c) Apple

* Glenn Fleishman is the author of dozens of books. His latest books, updated for iOS 14, iPadOS 14, and Big Sur, are Take Control of iOS and iPadOS Privacy and Security, Take Control of Your Apple ID, Second Edition, and Take Control of Wireless Networking and Security. He is a senior at Macworld, where he writes on Mac 911.

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