Wouldnt you want to use a much faster hard drive to run bitcoin on such as a "Samsung 960 Pro NVMe M.2 SSD" or something of that speed? Or would that not be able to connect to the Rasberry Pi? You would think you would want something faster than 100mb/s though, especially for a public node... those usb thumb drives arent very fast...
Aside from USB 3.0 limitation which already mentioned, external HDD is enough to run BTC full node unless you have hundred/thousand incoming connection.