Booting off an external drive and not seeing the internal drive

In preparation for upgrading to Catalina I decided to create an external drive running El Capitan so that I can run some old software that will not run on my current system, Mojave and won't run on Catalina. I am using an iMac 27 in. 5K, Late 2014. I have installed the software on the drive and have booted from the drive successfully.

The problem is I don't see the internal drive on the iMac as a drive icon on the El Capitan desktop. Also when I go to System Prefs>StartUp Disk I don't see the iMac drive to switch back to, which I guess is to be expected since it's not showing up on the desktop. I have to shut down the computer and let it boot up on the internal drive. Is there some networking settings that I need to implement that will allow the drive to be seen.

Thanks, Bob