Apple chose a different startup scheme for its Apple silicon models that prevents booting up from an Intel-targeted disk.
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.
Read more at MacWorld.com