Why won’t BIOS find the correct boot media?

My Win10 PC often goes through the BIOS boot-up and then tells me: “Reboot and Select Proper Boot device or Insert Boot Media in selected Boot device and press a key”.

If I Ctrl+Alt+Del it reboots fine. If I check BIOS settings and Save, it boots OK about five times then fails again. I have four HDDs for video content storage and a Samsung SSD with OS and apps.

I’ve updated the BIOS and fitted a new battery.

Any ideas?