I assume that you put the midi files NOT into a folder, you could do that in the floppy disk and was not a problem because of it's small capacity.
I found out the correct way to use the thumb drives, put files into folders that also help you to keep organized your files, I have one of 1 Gig, double of what you have and I have no trouble.
Just to be safe, let's review step by step.
1.-Format USB drive in Keyboard or if you wish to do it in the PC, just make sure that you choose the format FAT!!.....there is another, FAT 32.....that one NO.
2.-Create a folder in the thumb drive (example: Big band, or Frank Sinatra, etc..) then copy and paste the respective midis, in their folders....
3.-Do the same for styles, registrations, user voices, pads, etc.
I hope it helps.
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mdorantes
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mdorantes