The Apple M1 and Apple M2 Silicone Macs have a difference that has puzzled some readers over the way it was handled with previous Intel-based Macs: how to start 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 what is offered or affordable through Apple pricing. Or you want one as a backup in case something goes seriously wrong with your Mac M1 or M2.
Testing indicates that the following conditions are required to boot from an external volume:
USB-C: Use a USB-C connected drive, either a USB 3.1 or 3.2 drive or one that uses Ray 3 or Ray 4 natively.your best performance will come from using Thunderbolt 3 or 4 SSDs that use the NVMe standard (see below).
Delete before install:Erase the drive completely and then format it as APFS.
Install directly on the new volume:Get a macOS installer, and then install macOS from your M-series Mac directly to the external drive. (This will allow only an M1 or M2 Mac to boot from the drive; Intel Macs will not be able to boot from their Apple silicon-ready external drive.)
Let me expand on each point.
SSD over USB-C
With the first versions of macOS running on early M-series models, many people found that they had to use a native Thunderbolt 3 or 4 drive. Fortunately, macOS seems to have matured and can use USB 3.1 or 3.2 or Thunderbolt 3 or 4. Most cheap external drives use a version of USB 3 to connect via USB-C and rely on the slower SATA format, which together with an SSD it is about 10 times faster than a hard drive. You need Thunderbolt 3 or 4 to take advantage of the new NVMe/PCIe interface, which can be several times faster than a SATA SSD and two to three times more expensive. See this Mac 911 column for more details on the two interfaces and their performance.
(It’s possible to use a hard drive as an external M-series boot drive, but the performance will be so poor that even with a 7200rpm hard drive, you’ll wish you hadn’t.)
Erase and format as APFS
To use macOS 11 Big Sur (the first version to support the macOS M series) through macOS 13 Ventura (and future versions), the drive must be formatted as APFS. But tests by many people make it clear that you can’t just reformat 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 does not work properly, erase the drive before putting data on it. Start Disk Utility, select the SSD, click eraseand follow the prompts to create a single APFS container. This should remove 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 earlier than the current version, see this article on how to download older versions.
Launch the macOS 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.
If you want to make this drive a bootable clone, Bombich Software, makers of Carbon Copy Cloner, recommend that you first clone your data volume (which your software can do) and then install macOS after that.
Reboot from your internal drive or switch between
To return to your internal drive as the startup volume, you can open Startup Disk preferences while macOS is running on the external drive and select the internal drive. then click restart.
You’ll need to unmount the external drive after the reboot completes, and some people have reported that macOS says that one of your partitions is still in use. (Big Sur and later versions of macOS invisibly split a macOS into a volume containing system files and a volume containing your user data; the data volume may not unmount correctly.) You may prefer to power off at that point, unplug the external drive, power it back on, and plug it in.
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 one, where you can simply hold down the Option key while rebooting and select a drive (unless you’ve enabled certain security settings, in which case you’d need to use the mode recovery to disable them).
This is how you change the recovery mode boot drive with a Mac M1 or M2:
Select > to close.
When you see that your Mac has shut down, press and hold the power button until you see a message that says “Loading startup options.”
When the Options icon appears, you will also see a list of volumes next to it that you can select from. Select the volume you want to boot from.
Click Continue and the Mac reboots from that volume.
This Mac 911 article answers a question submitted by Macworld reader Gerald.
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