Made a mistake in my last post, I think CSM has to be enabled - will check my BIOS and get back to you.
CSM should be enabled and set to UEFI/Legacy (dual boot mode - not dual boot mode for OS's just to be clear!).
Also under Boot menu in BIOS should be Secure Boot - change it to 'Other OS'.
Swarfendor I've confirmed my BIOS mode is Legacy although I'll be danged if I can see how to change it if I wanted/needed to!
Also, I've found no place to select/deselect CSM much less anything about Secure Boot. I might be out of luck as my motherboard is simply too old? 2010 last updated in 2012. I'll look to see if there are any of the ASUS BIOS utility updates and get back with you.
I'm still mind boggled that this goof ball system will allow Zorin to boot up via the USB, but not from it's own HD in my computer. Dang it! All the best, PC
That's a good place to start. One of the first things I stated in my unofficial manual for Zorin 15 is that before you do anything else, update your BIOS! 
Many thanks. I will still poke around for a while for the single most obvious issue to me is that the motherboard etc will allow booting up via the USB which by all measure is just another "hard drive" as far as a computer sees it, but not the Hard Drive I've installed Zorin onto? It makes NO sense whatsoever. Again MANY thanks. Take care, PC
From your picture of your bios it looks like you have the same bios I have on my Asus M5A99FX motherboard. It doesn't show in your picture but just below where it shows the boot devices you should have a small window to click on to enter Advanced mode. From there you can turn off secure boot and check that CSM is turned on. I also had a lot of problems getting things to work until I changed the setting for my USB ports. You want them to be set up for UEFI/Legacy as swarfender has posted. Also I found that my USB ports were set for only partial initialization at start-up. You need to change that to full initialization. I hope this helps some.
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Citfta, thanks for your words. As an FYI this is what I've found.
Advanced mode is found under the Exit button at the upper right of my screen. Once I've clicked that here is what I see.
Clicking on the last choice, Advanced Mode, brings up several screen with one showing the status of the drives (Legacy)
BTW I've run Boot Repair from within the "booted" USB Zorin stick to no luck.
I then ran lsblk and got this report
OH, and I disconnected all hard drives except for the drive that has ONLY Zorin on it. The computer did not start and on failure it presented this screen.
Please understand, that the USB stick loaded with Zorin is the same stick that I used to "load" Zorin onto its own HD. I am not sure why the computer feels the installation has no boot device on it but I wouldn't know.
Everyone....THANKS so much for helping and I will continue to try my best to follow your instructions and guidance. PC
It wrote you lost efi partition. So you need creating a boot partition EFI. @Aravisian is expert from this and i know this problem exist here and described somewhere on this forum.
Have you tried with Legacy Disabled on both?
I will try both suggestions and reply back. TY all for these suggestions.
Edited title.
Good idea but changed nothing. Only lost the use of some of my USB ports for key boards, mouse etc. I put the choice back to Legacy Enabled.
Since all of this, I've removed all but the single 3tb hard drive I want Zorin to reside on. Removed all manual partitions, booted the computer up via the Zorin USB installation stick and then let Zorin install itself without any manual partion creations. Sadly, computer boots fine on the USB stick but still will not start from the HD.
I may start a new thread as I've given up trying to boot between HD's and OS's. I now have only the one HD with Zorin installed. Seems to be clean and the computer simply asks to reboot once the install from the USB stick has completed its job. But as mentioned from the start, the system refuses to boot from the HD.
Very strange that the computer starts on the USB but not on the HD.
Thoughts? Many thanks everyone. PC
Is this a Spinning HHD or an SSD?
If it is SSD, is it nVME or eMMC?
If it is spinning HDD, does the Boot Order Device have any Jumpers on it?
Aravisian, it is a spinning HHD. WD Part Number # WDBAAU0030HBK-01
I will try to find a mechanical boot order device but on the motherboard this drive is the first in the list for the system to look at when starting. I will look hard on the MB to see if there are any jumpers or dip switches.
TY for your suggestions, Pridanc
The jumper would be on the HDD, not the MB.
https://www.google.com/search?q=hdd+jumper&sxsrf=AOaemvKvYqMU2X3uPsyOgU-DXfZMZ2juIw:1643322314325&source=lnms&tbm=isch&sa=X&ved=2ahUKEwjzqa_W_NL1AhVJnWoFHfbKD_cQ_AUoAnoECAEQBA&biw=1418&bih=743&dpr=1
Folks, I've been here under dual boot hardware as I thought my issue was hardware. Long story short I've removed all HD's except the one that I have just installed Zorin on.
This time, I let the Zorin Install do its own thing without any manual partioning by me and it sure seemed to go smoothly. Once done I was told to reboot and did as well as removed the USB installation stick.
The computer came back with a "Reboot and select proper boot device". Hmm. I then went to the BIOS utility to tell it that I wanted the computer to start from my the new Zorin installation. Each time the computer comes back with the same words, yet, when I put the Installation USB stick back in I can get the computer to start from it in Zorin.
What is missing from the HD that keeps it from booting? Remember this is now the only HD on my computer.
I appreciate everyone's help on this. Sincerely, Pridanc
I wonder if you have not got a boot loader in place on your Zorin HD.
Can you run Gparted from the Zorin USB and check the partitions and boot loader flag that have been set up on that latest install.
Zaba, I will do that but what am I looking for? PLEASE understand I'm not computer savvy but can happily poke around with guidance.
Again, let me know what to look for or tell me to snag a snap shot after running Gparted to share with you.
All the best and Thanks! Pridanc
I merged the 3 posts from your new thread into this one.
As I understand it, your OP issue is still unresolved and removing the other drives is one of the many steps in this troubleshooting process. Best to keep this process together or it can confuse the ones trying to help you.
Here is a recent post that might help.
Pridanc, in that post he suggests making sure your sata drives are set to AHCI. Since your bios and mine are very close I think you will find that setting in the Advanced settings area.