Pleased to report that this is solved, which was done using the "accepted solution" from this thread.  Solved: How to repair broken EFI partition - Dell Community This is the same as the link in post 4 above, but the instructions were clearer on that thread.  So now my Inspiron has booted successfully from the HDD and I've got three years of Windows 10 updates to go through. It is also much much slower than with the SSD - I'd forgotten how wide the gap was though.   
My Inspiron 5570 shipped with a 1 TB HDD and Windows 10. I upgraded to Windows 11 a few years back and shortly after that I installed a 500GB M.2 SSD drive. Everything went fine and the machine boots from the SSD and I disabled the HDD via the BIOS as I didn't need it.  Fast forward to today and I want to revert to the HDD, delete my data from the SSD and then remove it.  Since this is UEFI there is no option to change the boot order in the BIOS. If I check the drives in Disk Management the SSD shows as "Boot" whereas the HDD does not. If I disable the SSD in the BIOS the machine won't boot because it doesn't see the HDD as a bootable drive. I must have done something to make the SSD bootable back when I installed it, which I now have to reverse to make the HDD the drive Windows boots from. But I can't remember what I need to do. So, how do I make the HDD the bootable drive and remove the "Boot" marker from the SSD?  [IMAGE]
Making original HDD bootable after SSD upgrade.