Upgrading to High Sierra: “You may not install this volume because the computer is missing a firmware partition”

If you try to upgrade to High Sierra (macOS 10.13) and get You may not install this volume because the computer is missing a firmware partition when trying to select your drive to upgrade, it may be because you're upgrading on an OWC drive.

If you're using Munki, the error may appear in your /Library/Managed Installs/Logs/Install.log as Starting macOS install: FAILED: startosinstall failed with return code 243.

Previously, you'd have to physically swap back the OEM drive, and then put the OWC drive back again, but now OWC has its own firmware updater tool that fixes the problem:
Aura SSDs: Firmware Update (beta).