> Unable to update external drive
I often start my M1 Mac mini up from a Thunderbolt SSD, but since updating its internal drive to macOS 11.3.1, I’ve been unable to update that external disk. How can I do that?
Using external startup drives with M1 Macs is tricky. To ensure success, you need to change security boot policy for the external drive in Recovery Mode.
Start your Mac up in Recovery by holding the Power button until the Options icon appears. Select Options, and click Continue to open the Recovery Assistant. There, select the user you know the password for, and click Next. Enter the password to see the Recovery window. In the Utilities menu, select Startup Security Utility. In its window, select the external disk with the old version of macOS on it and click Security Policy… Set the policy to Reduced Security and click OK, then restart your Mac using the Apple menu.
When your Mac restarts, open the Startup Disk pane and switch to your external drive. That should start up, and you can update it to the current release using the Software Update panel. Once that’s done, start up in Recovery Mode again and set boot policy for that drive back to Full Security, using Startup Security Utility to ensure your Mac is fully secure.