Apple’s M1 and M2 Apple silicone Macs have a difference that has made some readers stumble over the way they were treated with previous Intel-based Macs: how to boot or boot an M-series Mac from an external drive. Intel Macs generally make this easy.
You may want to use a bootable external drive to have a higher capacity SSD than is offered or affordable through Apple’s pricing. Or you’ll want one for backup in case something gets very pear-shaped with your M1 or M2 Mac.
Testing indicates that the following conditions are required to boot from an external volume:
USB-C: Use a USB-C connected drive, be it a USB 3.1 or 3.2 drive or one that uses Thunderbolt 3 or Thunderbolt 4 native.Your best performance will come from using Thunderbolt 3 or 4 SSD which uses NVMe standard (see below).
Clear before installation:Completely erase the drive and then format it as APFS.
Install directly on new volume:Get a macOS installer and then install macOS from your M-series Mac directly to the external drive. (As a result, only an M1 or M2 Mac can boot from the drive; Intel Macs cannot boot from your Apple silicone-prepared external drive.)
Let me elaborate on each point.
SSD over USB-C
With the first versions of macOS working on early M-series models, many people found themselves needing to use a native Thunderbolt 3 or 4 drive. Fortunately, macOS seems to have come of age and you can use USB 3.1 or 3.2 or Thunderbolt 3 or 4. Most inexpensive external drives use a flavor of USB 3 to connect via USB-C and rely on the slower SATA format, which when combined with an SSD is about 10 times faster than a hard drive. You need Thunderbolt 3 or 4 to take advantage of the newer NVMe/PCIe interface, which can be several times faster than a SATA SSD and about two to three times more expensive. For more information about the two interfaces and their throughput, see this Mac 911 column.
(It’s possible to use an HDD as an M-series external boot drive, but performance will be so poor, even with a 7200-rpm hard drive, that you’ll wish you hadn’t.)
Erase and format as APFS
To run macOS 11 Big Sur (the first compatible M-series version of macOS) through macOS 13 Ventura (and for future releases), the drive must be formatted as APFS. But testing by many people makes it clear that you can’t just change the formatting on an existing drive: invisible partitions used for purposes related to booting from an Intel drive from a previous macOS installation on the drive cause problems.
To avoid setting up a volume that doesn’t work properly, erase the drive before putting data on it. Launch Disk Utility, select the SSD, click To clearand follow the prompts to create a single APFS container. This should wipe out any conflicting data structures.
Install macOS on the external drive
Download the macOS installer directly from the Mac App Store. If you’re using a version of macOS that is older than the current release, see this article on how to download older releases.
Launch the macOS installer and select the external drive as the target. Follow the directions and steps. When your Mac reboots, it will boot from the external drive to complete the installation.
If you want to make a bootable clone of this drive, Bombich Software, makers of Carbon Copy Cloner, recommend that you clone your data volume first (which the software can do) and then install macOS after That.
Reboot from your internal drive or switch between
To go back to your internal drive as a boot volume, you can open the Startup Disk preference pane while macOS is running on the external drive and select the internal drive. Then click Restarting.
You will have to unmount the external drive after the reboot is complete, and some people have reported that macOS says one of the partitions remains in use. (Big Sur and later versions of macOS invisibly divide a macOS into a volume containing system files and a volume containing your user data; the data volume cannot be properly unmounted.) You may prefer to shut down at this point, the external disconnect the drive, reboot and connect.
You can also use recovery mode to change the startup disk. This is a bit more complicated with an M-series Mac than an Intel version, where you can just hold down the Option key while rebooting and select a drive (unless you have certain security settings enabled, in which case you use recovery mode to disable them).
To change the startup disk from recovery mode with an M1 or M2 Mac:
Select > Shut down.
When you see that your Mac is turned off, press and hold the power button until you see a prompt that says “Load Startup Options.”
When the Options icon appears, you will also see a list of volumes that you can select next to it. Select the volume you want to boot from.
Click Get on and the Mac will reboot from that volume.
This Mac 911 article answers a question from Macworld reader Gerald.
Ask Mac 911
We’ve put together a list of the most frequently asked questions, along with answers and links to columns: read our super frequently asked questions to see if your question is there. If not, we are always looking for new problems to solve! Email yours to [email protected] including screenshots where needed, and if you’d like to use your full name. Not every question is answered, we don’t answer email and we can’t provide direct advice to solve problems.