I believe I've found a solution. When I boot I press F12 to activate the one-time boot menu. It gives me options and I select the "USB Storage Device" option under "LEGACY BOOT". The first time it works, but the next time I try I get: Selected boot device failed. Press any key to reboot the system. It simply involves booting, pressing F12 to activate the one-time boot menu, and then selecting the option "BIOS Setup" under "OTHER OPTIONS". Then exit the setup, the server will reboot, and it will boot off that USB the next time!
This is a bizarre one. I boot into Linux from a USB flash drive. This USB stick was plugged into the top front USB port and booted (legacy mode) fine from that. Then one day the BIOS would report there was no bootable media available. So I moved the USB stick to the back far right port (viewing from behind). It booted fine. Then one day the BIOS would report there was no bootable media available. So I tried moving it to the port I'd been using before (front top) but still wouldn't boot. So I moved the USB stick to the back far left port (viewing from behind). It booted fine. I'm scared the BIOS is gradually freezing out USB ports I can boot from. I'm worried that soon I'll run out of USB ports I can boot from. Anybody else had this problem? What could it possibly be?
PowerEdge T20 Won't Boot USB Stick From Select USB Ports