Installing an older version of macOS onto an external drive

So I’ve got an oddly specific question.

I’m trying to install macOS Sierra onto a hard drive that I’m planning on installing into an old (potentially broken) 2011 iMac that currently does not have a hard drive. The problem is that the app from Apple’s installer can only be run on a Mac running Sierra (or earlier), which I do not possess.

Is there any way to install it onto the drive directly without an older Mac?

EDIT: I realize that more details of my scenario and what I have tried may be helpful.

My exact drive scenario is that I know the 2011 iMac does not have a drive inside of it. I have a 3.5" hard drive that I'm planning on installing, and an external SATA to USB enclosure. I want to install macOS on the drive first, because it's possible that something else is broken on the Mac, and I want to isolate it to not be something with a boot drive problem.

I have a 2018 MacBook Pro and access to a 2014 MacBook Pro (both running Catalina). I downloaded macOS Sierra from the support page I linked earlier, and ran the .pkg installer to get the Install macOS Sierra app (this only worked on the 2014 MBP, presumably because the 2018 shipped with Mojave).

Then, when attempting to open the Install app on either computer, I got the following error: This copy of the "Install macOS" application is too old to be opened on this version of macOS.

As such, not having access to a Mac running Sierra or earlier, I attempted to create a bootable drive, but that errored out with the following error:

Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Couldn't posix_spawn: error 35'

I don't exactly know where to go from here.

Solution 1:

I’m assuming your ultimate goal is to get that 2011 iMac up and running, but you’re experiencing trepidation about the prospect of replacing the internal disk drive because it’s behind a large glass screen.

You don’t need to install macOS onto an external drive to get things working.

• Create a USB installer. This model iMac supports Internet Recovery as well so this may be an option. I personally prefer to have and use offline USB installers.
• Replace the broken drive. The 2011 iMac’s glass screen is held in place with magnets and the display is secured with Torx screws. It’s actually an easy fix as described in this post (see TL;DR section)
• You may need the in-line temperature sensor to prevent your fans from spinning up to full speed because the sensor came with the hard drive in these models.

You will then be able to boot your 2011 iMac, install your OS with the USB installer or Internet Recovery and ultimately have a fully operational iMac.