Since this is an S905 board, it’s probably subject to all the weird and wonderful issues that I’ve gradually worked my way past on Potato and Frite. Before we go into those weeds, there is a possible issue with power draw with any external drive. I have some older (MLC era!) SSDs that I can use in an unpowered enclosure, but even laptop HDDs and many more modern SSDs may draw more power than can be reliably delivered through these USB ports, no matter the capacity of the upstream 5V power source.
So you say you’re using a separate power supply, or maybe even tried booting from a lowly flash drive and it still barfs? Join me as I recount my long, strange trip to getting USB booting to work… eventually.