Cannot install HS on iMac13,2 (Late 2012 27")

Howdy all:

I've tried multiple times, but cannot upgrade from Sierra or fresh-install High Sierra on my late 2012 27" iMac (iMac13,2). When I try either type of installation, first I get an error saying that the firmware couldn't be verified, and upon reboot I see an error that an installation package isn't available (OSsystem or OSinstall or something similar). I think the second error is a result of the first error but the installer trying to get past the first error.

Has anyone else with the same machine run into this? Have you been able to get past it?

I'm wondering if part of the problem may be due to booting off of an external drive--I'm using a Samsung SSD in a USB3 external enclosure as my startup disk because my internal HDD is dead. (NOTE: I was able to install HS to the drive when it was connected to my 2016 MBP, but I can't when it's connected to the 2012 iMac. When I then used the drive on my iMac, all was well until I had to update HS--I was back to a f***ed up installation on the iMac.)

Any tips for getting HS to install successfully on the iMac is greatly appreciated!