How to boot an Apple Silicon Mac from an external drive

Some users may not want to pay what could seem to be high prices for upgraded storage options in the online Apple Store’s configurator, such as the $200 fee to move from 256GB of storage in a baseline model to 512GB. Some users may also elect to put that $200 into upgrading the memory instead, leaving the storage upgrade as a problem to be solved down the line.

Others may have opted for a lower-capacity model just to get the Mac in their hands quicker, such as via a physical Apple Store, without considering their storage needs.

You can easily store a bootable external Thunderbolt 3 drive on the flat top of an M1 Mac mini.

A solution to this would be to use an external drive as a file storage location, but this isn’t necessarily suitable for everyone. There’s a lot of work involved in setting all apps to use that external storage, and many will simply prefer to deal with one drive instead of two or more.

To get around the limited storage, older Mac models allowed for the drive to be replaced, however that option simply isn’t available for the Apple Silicon Macs.

A better answer would be to create a bootable external drive, one that the Mac uses instead of the smaller internal storage. This guide will explain how to do it.

While this article is more about making it possible to have an M1-based Mac that has more primary storage, the same principle can be used to create a bootable drive for emergencies. If the main macOS installation fails, using an externally-bootable drive will allow a user to quickly get their Mac up and running without touching the internal storage, which can help with file retrieval and backing up before data is lost in a format.

For Intel-based Macs, there are already known processes that can be followed to create the bootable external drive, but the same instructions weren’t readily available for M1-based Macs until late December, following experimentation by users.

To create the external bootable drive, you will need a few things to get going. Firstly, you will need an M1-based Mac running on macOS Big Sur 11.1 or later, as earlier releases were problematic.

You will also need to have an external drive to boot from. As well as needing to be sufficient in capacity for your needs and fast, it also has to be compatible with the process itself, so you will need to use a drive that connects over Thunderbolt 3.

We need to stress that the process does require a Thunderbolt 3 drive to work, as you won’t have much luck with one using USB-C alone. It seems that something is preventing it from working with USB-C drives on a technical level, but Thunderbolt 3 drives work fine.

In researching the process, various Mac users have found success with non-Thunderbolt 3 drives, but under oddly specific circumstances without any real rhyme or reason for them working. Save yourself the time and just use a Thunderbolt 3 drive from the outset.

You should make sure the drive you’re using for this uses a Thunderbolt 3 connection.

This could take the form of a straight external hard drive that uses Thunderbolt 3, or a sufficiently-fast SSD or NVMe in a Thunderbolt 3 enclosure, or an external RAID enclosure that uses Thunderbolt 3. The key here is that it must connect using Thunderbolt 3.

This is a fairly lengthy process, so make sure your external drive is suitable before starting. The last thing you want is to go through all of this and discovering it fails halfway through because you used a USB-C drive instead of Thunderbolt 3, wasting your time and bandwidth in the process.

1. Open Disk Utility. This can be accessed within the Utilities folder in the Applications list.
2. Select the drive you want to use for the bootable external storage. Make sure it is the external drive and not the Mac’s internal drive, as this is the last chance to make sure.
3. Click Erase.
4. Click the Format drop-down box and select APFS. Make sure it is the standard version and not a variant with encryption.
5. Give the drive a name.
6. Click Erase, then Done.

There are two ways to actually install macOS Big Sur to the external drive. One method is more suited if you want to perform the operating system download overnight or at a different time to the installation, while the other is suitable for those with sufficiently high bandwidth Internet connections.

1. Open this Mac App Store link. If your browser asks, select to open it in the Mac App Store.
2. Click Get or the iCloud download icon to download it.
3. Open the Install macOS Big Sur app.
4. Click Continue in the installer, and follow the onscreen instructions carefully.
5. When the installer asks which drive to install macOS Big Sur to, select the APFS-formatted external drive that you want to make bootable, then Continue.
6. Proceed through the installer to the end.
7. When the Mac restarts, it will automatically boot from the external disk.

The Mac App Store listing for macOS Big Sur.

This method has users downloading macOS beforehand, then installing it to the external disk as a separate action. It is probably the more favorable of the methods due to freeing up the time for the download to when the Internet connection isn’t being used, such as at night or outside of business hours.

The second method, which follows, has users downloading the operating system and installing it as part of the process itself.

1. With the formatted external drive connected beforehand, turn off your Mac.
2. Turn the Mac back on by pressing and holding the power button, until the screen shows startup options.
3. Select the Options icon.
4. Enter an administrator’s user credentials.
5. Select Reinstall macOS Big Sur and follow the onscreen instructions carefully.
6. When the installer asks which drive to install macOS Big Sur to, select the APFS-formatted external drive that you want to make bootable, then Continue.
7. Proceed through the installer to the end.
8. When the Mac restarts, it will automatically boot from the external disk.

One of the menus you will see when installing macOS Big Sur to the external drive.

Both methods leave your Mac with a fresh macOS installation, running off the external bootable drive. You will have to complete the initial Mac setup process, but once that is complete and the desktop is accessible, you can shut down the Mac and disconnect the drive safely.

When you turn the Mac back on, with the bootable drive disconnected, it should automatically select the internal drive and load normally. If not, turn it on with a long press of the power button to bring up the startup options, and select the primary drive.

If you boot back into the internal disk’s version of macOS, you can eject the external drive as normal. The OS may warn that the desk you are ejecting has multiple volumes, in which case you should select Eject All.

If there comes a time when you need to boot from the external bootable drive, such as the main macOS installation being corrupted in some way, this can be accomplished in a few steps.

1. With the Mac powered off, connect the external bootable drive to the Thunderbolt 3 port.
2. Turn the Mac back on with a long press of the power button, holding until the screen shows startup options.
3. Select the bootable external drive.
4. Your Mac will then boot from the external drive instead of the internal storage.

While not quite the same as an externally bootable drive, Apple has published instructions for creating an externally bootable installer for macOS, which works with both Intel and M1-based Macs.

You could also reinstall macOS to the internal drive instead of the external drive using the second method by selecting the internal drive, a method we have covered previously. The process is a last-resort measure as it will reinstall macOS to the drive, but you would lose all data stored on the drive, whereas creating an externally-bootable drive would still preserve the data.