Have you tried the the usbs by saving something on them?
I downloaded a list from Yamaha site that listed the USB’s that had been tested for use ( or something along those lines also I think they can only be up to 32 gig, if I remember correctly. I had a couple of 16 gig sandisks the type I had weren’t mentioned on the list , but they did work.

When I did my update I must have been having a bad morning, haha, or nervous about doing update, I didn’t quite understand the

C. Extract the *.prg file and copy it to the root directory of your USB flash drive.

I did the extraction, then,
Wasn’t sure if it was supposed to be left in the extracted folder or just the file inside the folder.
I had both on there, went thru procedure and it worked as it should.
Still haven’t got a clue which was the correct one.