Quick post here, as there is a quick solution for most people having the error “ROM file is not an EFI BIOS”. I recently had this with my Asus motherboard. Even though I had double checked that I had downloaded the correct BIOS update from their support website, I received this error when I tried from my USB as well as my hard drive.
Here’s what I found out about the issue. The device being read from to get the ROM file must be formatted as FAT32, not NTFS. While you can navigate file and see the file within the EFI BIOS, the underlying EZ-Flash program is separate and only reads FAT32 (and possible FAT).
So format your USB stick as FAT32, drop the file on, and try that.