I have the above MBA upgraded from the original 128GB Flash Drive to the present OWC AURA SSD, some 2 years ago and it has been working fine with macOS 10.12.6 Sierra. It is also encrypted upon power up before loading the OS for travelling protection. I tried to update to macOS High Sierra but cannot as it says I need a firmware update. I have read before to reinstall the original drive to allow the firmware update, but I no longer have that. What can I do now?