USB not connected

I ran into the same problem of losing connectivity to my Prusa USB thumb drive on my MK4s printer. I could read the drive when plugged into my laptop with no problem, so the problem was not with the drive itself. To correct the problem, I used an industry standard electronics aerosol cleaner on the printer LCD connections for the USB, ground cable and data ribbon cables. After reconnecting everything, and powering up the printer, the Prusa USB thumb drive was accessible again. The bottom line is that the quality of the electrical contacts, on the Prusa LCD board, are not as good as they used to be when I started with the original Mk3 many years ago, which has been upgraded several times. Occasional cleaning of the on board and peripheral insert connectors is required to maintain proper data and electrical conductivity.

This happened to me on my MK3.5S upgrade. It turned out the ribbon cable was not totally seated on the LCD board. After plugging it in fully, the USB drive was recognized and I could update firmware.