Is there a file somewhere in the System folder or in the Library that shows me which OS is installed on the disk?
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3 Answers
You can read the SystemVersion.plist on that particular volume-
/Volumes/<volume_name>/System/Library/CoreServices/SystemVersion.plist 
If there's a valid, bootable system on it, then it will be listed in System Preferences > Startup Disk, with its version number.
- Doesn't work for me. I see no system info for the external disks.SPRBRN– SPRBRN2020-08-15 11:55:36 +00:00Commented Aug 15, 2020 at 11:55
- @SPRBRN That's odd. See the screenshot I've added.benwiggy– benwiggy2020-08-15 12:38:09 +00:00Commented Aug 15, 2020 at 12:38
- I see, but here it only shows the volume name.SPRBRN– SPRBRN2020-08-15 14:47:30 +00:00Commented Aug 15, 2020 at 14:47
- 1Could it mean those volumes are not bootable?Steve Chambers– Steve Chambers2020-08-15 16:05:06 +00:00Commented Aug 15, 2020 at 16:05
- 2@SteveChambers - volumes are bootable, but encrypted. Maybe that's the issue here?SPRBRN– SPRBRN2020-08-16 15:10:40 +00:00Commented Aug 16, 2020 at 15:10
On a Mac, you can use the defaults read command to get just the "ProductVersion" from the "SystemVersion.plist" file.
defaults read /Volumes/"$Your_disk_name"/System/Library/CoreServices/SystemVersion.plist ProductVersion To read it from a non-Mac machine, just look at the content of that System/Library/CoreServices/SystemVersion.plist file, which is a plain text XML file.
Or on Linux, you can do
grep -A1 ProductVersion "$Your_disk_mount_point"/System/Library/CoreServices/SystemVersion.plist 