How to tell which disk was used to startup macOS

[scuac]

Ok, this may be a very niche problem, but imagine I have a system with multiple drives, each capable of starting up their own version of macOS (but the same version number) during boot time. Once the OS is up and running, how can you tell from which drive the OS was started?

 

[Mike Boreham]

Click the apple symbol top left of screen. Then "About this Mac"

The Start up disk is fourth item in the panel which appears.

 

[DeltaMac]

The storage drive partition that you are currently booted (the Startup Disk) will be listed in About this Mac.
The boot drive that you have selected in your System Preferences/Startup Disk will be the one that boots your system (assuming that the hardware is working OK, is compatible with your Mac, and is installed properly.)

 

[scuac]

Click the apple symbol top left of screen. Then "About this Mac"

The Start up disk is fourth item in the panel which appears.

Does that show the disk that was booted from or the disk that is setup to boot from by default? Slight difference, e.g. if I have drives A,B,C and I have things setup to boot from A by default and then I use the press Option during boot to select drive B, will this show drive B or the drive A (which is the one configured to boot from by default)?

The storage drive partition that you are currently booted (the Startup Disk) will be listed in About this Mac.
The boot drive that you have selected in your System Preferences/Startup Disk will be the one that boots your system (assuming that the hardware is working OK, is compatible with your Mac, and is installed properly.)

It is possible to boot from a drive different than the one listed in Preferences/Startup Disk by pressing Option during boot.

 

[Mike Boreham]

About this Mac shows the volume (you might have several bootable volumes on the same drive) you are actually booted from.

Yes option booting shows a row of bootable volumes and you can chose there. You can let go of the option key when the white apple appears.

 

[DeltaMac]

And, the option-boot manager will show connected bootable volumes, but don't assume that all are capable of booting that Mac. Some may appear that are too old (or too new) to boot.
The Apple system boot icon can be white or black. I have a mini that shows the older black (on white) Apple icon when booting to Mountain Lion, but the Apple icon shows white on a black screen when booting to Yosemite or newer.