Just reinstalled FACT on to a second drive which an SSD and am getting the error in the screenshot. I have uninstalled this and reinstalled but still getting the same error.
That directory stores our airport ALT (altitude) files. It's created automatically by the installer. You can check if it exists. If it doesn't it should be created if you run the installer again.
We haven't had any reports of that problem yet, independently on where you install the addon (HDD, SSD, external,...).