Which Mac OS is Installed on external drive?

[Jodeo]

I have an external hard disk drive with two partitions, each with an install of Mac OS.

Either Yosemite or El Capitan is installed on either (maybe both) partitions.

Is there an easy way to tell without actually booting from each? In other words, what could I look for in each partition's directory for an indication of which OS it is? (I know I can boot and check from the Apple > About This Mac menu option.)

The drive is connected via USB2 to an old iMac and reboots are slow.



Thanks
[doublepost=1531880596][/doublepost]...annnnnnd I just found two answers.

1: In Finder, navigate to [root]/System/Library/CoreServices/SystemVersion.plist and press spacebar to view the file. The OS version is there, and

2: Go to Apple Menu > System Preferences... > Start Up Disk. If the drives have the ability to boot up with the OS, the version is shown.

So there's that. Which is nice. Now, on to some bacon...

 

[chrfr]

I have an external hard disk drive with two partitions, each with an install of Mac OS.

Either Yosemite or El Capitan is installed on either (maybe both) partitions.

Is there an easy way to tell without actually booting from each? In other words, what could I look for in each partition's directory for an indication of which OS it is? (I know I can boot and check from the Apple > About This Mac menu option.)

The drive is connected via USB2 to an old iMac and reboots are slow.



Thanks
[doublepost=1531880596][/doublepost]...annnnnnd I just found two answers.

1: In Finder, navigate to [root]/System/Library/CoreServices/SystemVersion.plist and press spacebar to view the file. The OS version is there, and

2: Go to Apple Menu > System Preferences... > Start Up Disk. If the drives have the ability to boot up with the OS, the version is shown.

So there's that. Which is nice. Now, on to some bacon...

You can also get info on each disk and you’ll see the version there.

 

[Jodeo]

OK, new mystery: One partition has a clone of the 10.10.5 (Yosemite) internal drive in my old iMac. The other partition had a clean install of El Capitan, updated, with the internal drive migrated to it (NOT cloned; I just set it up to run for a day or so as USB 2 is so slow).

Both partitions on the external drive are selectable as the Start-Up Disk (though System Preferences and through boot+Command key).

However, no matter how I try to boot from the external Yosemite drive, it always boots to the El Capitan partition - either as the selected Start-Up Disk or when I restart and hold Command after the chime (remember the chime?).

I wanted to do this so that I could...
1. Wipe the internal drive, and
2. Clone the El Capitan partition to the internal drive.

I know I could do this in other ways (e.g. boot from the recovery volume), but I want to ensure the external Yosemite partition WORKS just in case I decide later to downgrade the internal drive back to Yosemite - by cloning the external Yosemite partition back to the iMac.

So.
How can I test the Yosemite (external) drive w/o the computer going over to the El Capitan partition on the same hard drive?

BTW: iMac mid-2007 al, 20" 2.4Ghz, 6GB. I've read that El Capitan actually runs (slightly) better on these older iMacs than Yosemite. (And yes, I know I need a newer Mac, too.)