Computer is now booting to the UEFI instead of directly to Windows

I had this same problem this very morning.
I had just added an additional Hard drive, just an easy straight forward task.

I was amazed that the PC would not boot - and the Gigabyte set up page appeared inferring that the machine had unsuccessfully tried to boot using UEFI. I was asked to go into the boot settings and confirm which drive should be chosen. I could not find the disk listed! This of course gave me the info needed to work the problem out.

A quick check revealed that I had knocked the sata lead on the C:\ drive - so I pushed it back (just a very minimal distance), and immediate success.
I do feel that the usual mass produced sata leads leave something to be desired.
This may not be the answer in your case, but, it may help someone who has played about inside (as I did) and then experienced a non start.
Regards
C

Here are the IRST settings and HD BBS Priorities, in order.

Try disabling IRST.

Disabling IRST didn't work.

Where is the screenshot of Disk Mgmt requested?

Highlight Boot Failure Guard and Count listings to see what it says they are on the right hand side. By now you should have looked at this text explanation of every setting in the BIOS to see if they apply. What exactly is the Boot Failure Guard?

I still think it might be Setup Prompt Timeout. What does the explanation for that say?

Does the popup at boot include any other error or box or just the Main page of the BIOS setup that you showed?

Go to the AsRock Support Downloads webpage for your model mobo and read the Manual to try to find how to suppress the popup you are getting. If it's not in the Manual then contact their Support to ask about it.

There is no description for what Boot Failure Guard is. Here is the description for Setup Prompt Timeout:

"Number of seconds to wait for setup activation key. 65535(0xFFFF) means indefinite waiting."

Yeah, it just boots to the main page of the UEFI setup utility.

Here's the requested image of the Disk Mgmt:

So what are the settings choices for those two settings?