How to create "firmware partition" on HDD?

I have a problem with internal hard drive in my IMac, it doesn't work, but I have an external hard drive (HDD), and I want to run MacOS on it.

I successfully installed MacOS and upgraded it to Sierra. The problem is that I can't update to High Sierra, or Mojave, because the hard drive doesn't have "firmware partition".

How can I create "firmware partition" on a simple external HDD?

I spend all day searching for info online, but found nothing helpful in my case, I would be very grateful for the help! Thank you in advance!

Here is the diskutil list log:

Solution 1:

You must have an EFI partition (aka firmware partition) on an internal drive for the installer to accept that. You have not provided the model of your iMac, so that might be an issue. Basically, if you have one of the older iMacs, you could replace the hard drive with an SSD. 256 GB SSD are pretty cheap, have a little room for stuff and are nice and fast.

Solution 2:

There have been reports of problems when the internal drive is missing or broken, that High Sierra (10.13.4 and above) and Mojave won't install onto external drives, because it wants to update the EFI firmware partition on the internal drive.

Third-party internal replacements, which are 'blank', are also affected.

Solutions include:

• installing MacOS onto your external using another Mac that has a working internal drive.
• Replacing the internal drive.

There may be other ways of creating a Mojave installer that doesn't check for the internal EFI.