boot from hdd in caddy

Out of my depth here - Inspiron 15 3580 with SSD main drive. I've installed a HDD in the optical drive caddy, formatted and initialised it and set it to receive file history in Windows 11, all working fine.  After a few wakeups from sleep Dell pre-boot system performance check starts "initialising storage", then system scan, finds no issues, goes into OS recovery, but then finds no boot drive on restart. If I remove the HDD in the caddy BIOS starts and Windows 11 opens and runs normally. 

Do I need to change the boot sequence in BIOS, if so I can't see how to do that. I've seen suggestions just to swap the hard drives but I'd rather not. Can anyone suggest a solution please?