After purchasing an Apple silicon M1-based Mac, people have discovered that an external startup volume used with an Intel Mac won't boot the newer computer. Apple has organized hidden partitions that are part of the startup process differently for macOS volumes that handle Intel and M1 Macs. As far as anyone understands and has attempted, you can't make a dual Intel/M1 bootable startup drive. (If you know otherwise, let us know.)
That's generally not that big of a problem. Most people who aren't system administrators rarely need a way to start up a computer externally when that drive isn't set up as the default startup drive. The situation asserts itself mostly when you want to migrate from an Intel Mac to an M1 Mac while using the same external drive and you didn't purchase a Mac with an internal volume large enough to use Migration Assistant to hold your external startup's contents.
This happened to me recently. I had upgraded my 2017 Intel iMac to an external Thunderbolt 3 SSD in 2020 because its internal Fusion drive couldn't keep up with my work. It made a huge difference in performance. When the iMac's motherboard died a few weeks ago, well out of warranty, I shifted to an M1 Mac mini with a tiny internal drive, as I already had a high-performance 1TB SSD and didn't want to pay the high cost for that capacity with the Mac mini.
However, like some readers who have written in about the situation, I couldn't boot from my Intel-targeted external drive. I wanted to upgrade the external drive to work with an M1, and that required performing a little dance with an additional drive of high-enough capacity to store everything from my external startup drive.
This operation is simplified by the system/Data volume split in macOS 10.15 Catalina and later. Instead of a single startup volume that has all the system and data files, Apple broke macOS into two pieces for greater security. When you clone Big Sur (or the upcoming Monterey) to migrate it in this process, you only need to copy the Data volume, as the system volume has to be created as part of the macOS installation or upgrade process.
There are two paths you can take. But before you start down either, make sure you have a full clone of your Intel Mac's Data volume. If you're already using Time Machine, you should still make an additional copy because Time Machine is fragile and sometimes unreliable.
I highly recommend using a Time Machine backup for this external-drive system switchover. In my testing, the Time Machine path seemed to work better than relying on a disk clone of an Intel startup volume—at least a couple of months ago. Apple keeps improving its migration tools, but macOS clearly likes its own backup format best.
If you choose the path to make a clone, I prefer Carbon Copy Cloner to make an identical copy, but Disk Utility will do the trick, too.
If you're using an SSD as the startup volume, you don't need to buy another SSD for this task if you don't already own one. You could purchase an inexpensive, high-capacity hard disk drive (HDD). Although using an HDD increases the time both to make a clone and restore it, the cost is so much lower than an SSD, and then you have that HDD available for Time Machine backups afterwards.
Here's how to perform the migration:
- Complete any necessary basic setup for your new M1 Mac so that you can run macOS. You need an account set up to complete the next step.
- After logging into the macOS on the M1, shift the M1 Mac into recoveryOS: Shut down the M1 Mac ( > Shut Down). Hold down the power button for around 10 seconds. When you see Loading startup options, release the button and wait for the Options icon to appear. Click it and authenticate if prompted.
- When the macOS Recovery dialog appears, choose Utilities > Disk Utility.
- Choose View > Show All Devices.
- Select your external drive under the list of External drives in the sidebar. (Choose View > Show Sidebar if it doesn't appear.) Select the main drive, not any volumes beneath it.
- Click Erase. Name the drive and make sure Format is set to APFS and Scheme to GUID Partition Map. Click Erase again to confirm the erasure. Warning: At this point, all data on the external drive will be gone forever.
- Quit Disk Utility, which will return you to macOS Recovery.
- Click Reinstall macOS Big Sur (or a later macOS). Follow the prompts to select the external drive and complete the installation on it of macOS.
- When the installation is complete, your M1 Mac restarts and begins the final set-up stages. When you reach the Migration Assistant screen, select From a Mac, Time Machine backup, or Startup disk. Click Continue.
- You can now select your Time Machine backup from an attached disk or over a local network, or choose the clone you made before erasing the SSD.
The migration can take a long time, as you'll have experienced if you've gone through it before. The 800GB of files I had took a few hours to drop into place. But when the migration was complete and my new M1 Mac rebooted, I was dropped right back into the moment my Intel Mac had died—practically down to the minute.
This Mac 911 article is in response to a question submitted by a Macworld reader.
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