Roms not showing on Picade 2015

Hopefully someone can help with this. I have a Picade 2015 edition which I have loaded with Retropie for a Raspberry PI 3 Model B v.12. I then followed these instructions: Transferring Roms - RetroPie Docs.

I installed the SD card and after powering on it takes me to the Configuration screen after mapping the buttons. I created a USB drive following the standard instructions to format as FAT32 and create a single folder call Retropie and the inserted into the Picade. Left for a few minutes and then removed and inserted back into my laptop. I downloaded Donkey Kong 64 to just test the process.

I restarted the Picade but nothing is showing after the EmulatorStation has loaded.

I tried to transfer the files using File Manager but CANNOT see the USB mounted on the media path.

Any ideas please?

Many thanks,