How do I determine an external drive's macOS version?

In AppleScript, without having to launch Disk Utility this will allow you to pick what is in the /Volumes folder (in particular, a mounted external drive) and display the information shown in the dialog below. Just copy/paste into the Script Editor, click the compile button, and run… I don't have an external bootable drive with me (traveling) to test that functionality.

use framework "Foundation"
use AppleScript version "2.4" -- yosemite or later
use scripting additions

property NSDictionary : a reference to current application's NSDictionary
property SysVersion : "Macintosh HD/System/Library/CoreServices/SystemVersion.plist"

set VolPath to POSIX path of (choose folder default location (POSIX file "/Volumes" as alias)) as text

if VolPath = "/" then
	set fullPath to (VolPath & text 14 thru -1 of SysVersion)
else
	set fullPath to POSIX path of (VolPath & SysVersion) as text
end if

set dict to NSDictionary's dictionaryWithContentsOfFile:fullPath
set theOS to (dict's valueForKey:"ProductUserVisibleVersion") as text
set buildVersion to (dict's valueForKey:"ProductBuildVersion") as text
display dialog "macOS " & theOS & " (" & buildVersion & ")" with title "macOS Drive Version"
return

Open Disk Utility, and select the drive.

What did you use to create the bootable drive to begin with?

You won’t be able to boot a Mac studio from a boot drive created using an Intel based Mac regardless of what version it is.

I don't know the answer to your question but, did you allow external booting in Startup Security Utility?

About Startup Security Utility on a Mac with the Apple T2 Security Chip - Apple Support

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