Background: I have a mid-2013 MBA I was given a couple of years ago without its SSD. I used it with an external USB 3.0 HD and upgraded the OS each new version up to Sierra, but could never get High Sierra to install.
Now I've just bought a cheap external USB 3.0 SSD to get a speed boost and I've installed a fresh OS via the Internet Recovery. It only allowed me to install Mountain Lion, which I guess was the current OS when the MBA was new.
I've gone to the App store to update to Mojave, the only version they seem to offer. I download and ran the update and when it's time to choose the drive to install on, my new SSD, it told me "You may not install to this volume because the computer is missing a firmware partition."
One website said to install the firmware update from Apple's support site, but running it tells me I'm already up to date.
Some websites make it look like the only ways to solve similar problems requires the OEM SSD, which I have no access to.
I would prefer to have the latest OS but I'd settle for Sierra if it's utterly impossible to install anything newer. But I can only find Mojave on the App store.
(I do still have my external HD with Sierra installed but I thought a clean install would have some benefits.)
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *120.0 GB disk1
1: EFI 209.7 MB disk1s1
2: Apple_HFS Mac USB SSD 119.2 GB disk1s2
3: Apple_Boot Recovery HD 650.0 MB disk1s3