No longer booting from USB

Yesterday I was able to boot (a non-Windows/non-Linux OS) from a USB stick. No problem. I then tried to boot the same image from the SD card, configured exactly like the USB stick, and got Couldn't get UEFI db list. The BIOS was set up properly. Figured I'd just go back to booting from my USB stick but I still get the same message and linux ends up booting. I didn't change anything on the USB stick and I've also tried resetting the BIOS to the defaults.
Since the exact USB stick, untouched, booted fine yesterday I assume it's some kind of BIOS config? Anyone have any ideas? Thanks.

Not sure if this was it but resetting/Golding the RTC got everything back to default (i had already reset the BIOS) and booting off USB again.

Thank you! I couldn't get my Atomic Pi to boot from USB (it would keep booting to Lubuntu) but pressing the RTC reset button solved my issue! Thank you again!!