Comment 16 for bug 1423830

I had the same error as #5 installing Ubuntu 18.04

I ended up finding a solution from an SO post from 2015 (!)

https://askubuntu.com/questions/502307/the-attempt-to-mount-a-file-system-with-type-vfat-in-ssi10-0-0-partition1sda

mkfs.vfat /dev/sdb1 worked for me to format the EFI partition and the installer worked when I rebooted and tried it again