I took any old USB stick I have been using (although I only use Sandisk)
Unzipped using WINRAR Mobile app. the downloaded .zip from my mobile smartphone through the connected OTG cable/USB stick
Made sure the extracted file was on ROOT of drive and was around 290Mb (as I have MANY other folders on root)
Plugged, restarted Genos, holding reqd buttons.
Update file found straight away, completed upgrade after 2 minutes.
This is just to illustrate that the File System/OS is not fussy, so long as a clean file.
What I DID do was to make sure update file is on ROOT of the drive, and check the display
to ensure it agreed update file found.
I also unplugged all other USB's in use (the one underneath) before update and rebooting.
You really want to reboot with NO USB's occupied first, then reboot with the upgrade stick in.
Chrs!