As mention by others above, the FLXS desk don't update with a bootable USB drive, you just copy the update file to the USB root directory and on the desk go into Setup > Load and select the .exe from the list of files and tap the load option.
If you are having issue seeing the file on drive check you can see the file as ZerOS7.13.exe on a PC, in case a virus protection/firewall has change it.
IF you can't see the USB drive, as you mention make sure it Fat32 or if a large capacity drive NTFS format, find windows tends to default to Ex-Fat.
If you are still having issues PM me on the address in my signature.