Boot up problem, Dell E7470 Win10.

For months, I've run the laptop with a USB stick plugged in, with no problems when booting.
But now, it tries to boot from the non-bootable USB stick! This gives the inevitable 'Boot manager missing' message. I have to reboot & hold f12 to boot from the internal drive.

I could remove the stick & insert it after boot up, but this annoyance wasn't there before! The BIOS has always been set to try a USB port first, then the internal drive. But why does it try to boot from an obviously non-bootable drive?

Any ideas for fixing this?

Latitude E7470, made 2017, Intel i5, W10 Pro build 20H2, 256GB SSD M.2 2280, 128GB SD card for portable apps etc, 16 GB ram

Thanks for any advice. Kind regards, Niloc1017