Where I work, there is an iMac Pro with a broken hard drive. My company doesn't want to dish out money to fix it, so I took it and installed Mac OS High Sierra on an external hard drive and booted from it. Everything works well even though there is some lag due to the USB connection.
I'm unable to install Mojave update due to "you may not install to this volume because it is missing a firmware partition" error. I have tried erasing and reinstalling Mojave from recovery but it doesn't work as well.
I tried installing Mojave on the hard drive using my Macbook and installed and booted fine, however, when I try to boot the iMac from it, it doesn't work.
I'm not sure what else to do. I also tried installing Sierra then updating to High Sierra, however that didn't do any firmware updates.
Below is the iMac specifications:
Model Name: iMac
Model Identifier: iMac15,1
Processor Name: Intel Core i5
Processor Speed: 3.5 GHz
Number of Processors: 1
Total Number of Cores: 4
L2 Cache (per Core): 256 KB
L3 Cache: 6 MB
Memory: 16 GB
Boot ROM Version: IM151.0207.B07
SMC Version (system): 2.23f11
Best Answer
Having come across a similar error myself, I found that it won't install Mojave onto an external hard disk due its need for the EFI partition to be on a local disk. The EFI partition is only 200MB+, so in theory you could get away with installing a very small hard-drive internally and just stick the EFI partition there. Once Mojave's installed, you can move the EFI partition to a USB (probably needs to be placed before any other partitions).
I use Linux/Gparted to change partitions for this purpose. You can do that entirely from e.g. the Ubuntu install USB without having to install Linux at all. You probably have an EFI partition somewhere on your USB drive, so simply copy it to an internal hard-drive. Since it's probably only 250MB, you can get such drives for extremely cheap off eBay if you don't have a drive lying around somewhere from an archaic machine.