Hi Crispin.
Please plug your USB stick back into your PC and then from File Manager right click on it and select to look at its properties to check that it is indeed formatted as FAT (or FAT32)
NOTE, when you plug it into windows, windows may say there is something wrong with it and offer to fix it. DONT DO that and if you did (last time) please reformat the USB on the B2.
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If the USB is FAT formatted, remove it, and plug it into the B2's USB C port and power the B2 up.
Next select an empty playlist using the B2's Web UI, open it and
Then select an artist from the central panel area then click on the 3 dots on an album in the top left area

and select "Add to Playlist" and choose the name of the play list you want to add this to. (In my case "green").
the playlist will then look like this.
OK you now have music in your test playlist. You now need to move to your B2's front panel and using the menus
do this command
When it is finished (not long) please then press and hold in the "Play/Stop" button on the B2 until "saving appears on the front display. Then release the button and wait until the clock appears. Power the B2 off and then on again.
After it has booted, go to the Web UI and select USB C - this should show you what you have put on the USB stick.
Hope this helps,
Fred