Solved: bootable ssd big sur m1 apple silicon

Creating bootable external ssd  macos big sur m1 apple silicon

I installed macos Big Sur 11.6.8 on an external 256GB m.2 NVME ssd connected to the Thunderbolt port on an m1 Apple Silicon machine.  I used both the standalone installer downloaded from the App Store and from macos Recovery mode (holding down the power button).

Each time install would fail at the first reboot of the installer.

The install log in the Installer gives a hint:

 bridge device not found 

Problem:

 

The ssd is bus powered; it is not powered up when the installer restarts.

Solution:

The external ssd needs an external power source.  I connected the ssd to a port on a drive dock with its own power brick.  After installation you can plug the external ssd into the Thunderbolt port again.  

In addition, remember to set your internal drive as the startup drive (Apple menu> system prefs>Startup Disk) when you disconnect your external ssd.  Your system may not start correctly if you don't set the startup disk.

This applies to macos Big Sur 11.6.8.