Back again, hello everyone.
After I added a GPT partition to my new 4TB Samsung Evo I went ahead and tried to prepare it properly in order to be able to install MX Linux 21.3 on it. For the installation I utilize a Live MX USB stick. This is something that I've done for years, probably installed MX Linux over 100 times on many different machines, but always on disks with MBR on them.

According to documentation that I located Online I created a 512 MB FAT32 boot partition as SDA1 and utilized the flag option to mark that partition as the boot partition.

Aside from a swap partition there is only one other 225GB partition which was created and formatted as ext4 in order to be able to install the system with root privileges.

The installation process began initially without a problem, except for the fact that a message appeared informing me of the missing MBR for which GRUB would be required. Proceed anyway? To which I responded, yes. I proceeded anyway because I had created the required EFI / FAT32 boot partition and figured that the MX installer would make use of that automatically. Sadly that wasn't the case. Upon rebooting the system an error appeared, boot disk or operating system couldn't be found.

So I figured perhaps it had something to do with my Bios settings. I made adjustments for disable legacy mode but keep secureboot enabled ... which did not work. Same message about missing boot disk or missing operating system. So I got back into the bios and changed the setting to enable legacy but keep secureboot disabled. Tried booting into the system again, same problem. My bios has the 4TB disk showing accurately (actually 3.73 TB). It is also selected to be the first boot device by default. So at this point I don't have a clue as to what I need to do in order to be able to boot my newly installed system?
Any helpful thoughts would be appreciated.

Using gparted or some similar software, you need to set your 512MB vfat partition as boot and efi. In gparted, right click the EFI partition in the main window and move down and select Manage flags and select the check box to the left of boot and efi. Did you do that? Did you boot the usb to install in EFI mode? The fact that the installer asked about the MBR would seem to indicate it was not booted EFI as the MBR is not used with EFI boot code.

You can install in Legacy/MBR mode on a GPT disk but then you would need to create an unformatted 1MB bios_boot partition and have that partition set to bios_boot with manage flags. The bios_boot partiton is needed in this case for the Grub core.img file. Better to boot and install EFI.

Yes, everything was done as you suggested and as I had read elsewhere. But you're right, when I booted into the Live MX installation USB I did not use EFI. Didn't think that it would matter for the USB stick. I'll try again tomorrow morning (it's evening time here). Thank you.

There is a boot repair in the MX menu.

Wasn't certain if Boot Repair could work with the differences between a legacy boot stick alongside a UEFI required installation. So just to play it safe I went ahead and booted back into the USB installation mode, this time using the UEFI option. Then I reinstalled the system. Right away I could see that the boot (menu?) was being installed correctly into the root ESP partition which was created for that purpose. So this time around the installation went fine, after a reboot the system was there as it should be, without any errors.

Just for the heck of it I did try to enable fastboot & secureboot in the bios but that didn't work. Received drive not found error. As soon as I went back into the bios, disabling fastboot as well as secureboot, everything was fine again.