I successfully flashed my ver 3402 bios to a modified ver 3802N using the excellent guide provided by Fernando at the beginning of this thread and installed Win10 Home on a new 256GB M.2 NVMe SSD mounted on a PCIe adapter. I was able to install Win10 on the SSD from a USB stick created with Rufus software and then boot Win10 off the SSD using the UEFI Compatible ROM setting in the bios. Everything worked fine!!!

My next logical step was to clone the HDD OS partition to the SSD OS partition. When I reattached by internal HDD (with the original Legacy boot OS still on it) and tried to reboot from the SSD still in UEFI Compatible ROM mode, the bios would not post. I disconnected the HDD, rebooted to the bios and set my bios SATA Configuration for the HDD SATA interface to "Hot Plug". After I successfully rebooted to Win10 on the SSD, I reconnected the HDD and about 3 mins later, the HDD was loaded and accessible as Drive E:

Since I wanted to clone the old MBR OS partition to the new SSD OS partition I cloned the old OS HDD partition to another partition on the HDD and changed the HDD from MBR to GPT. My "Plan - Part 2" was to reboot the system and select the now GPT HDD using the "One Time Boot" with the UEFI Compatible ROM setting in the bios, copy the cloned OS partition to the SSD OS partition and then reboot to the SSD as before. AND this is where the problem occurred..... I could never get back to booting from the HDD.... Or anything else.....

On startup, everything powered up and the CPU,VGA & Memory trouble l.e.d.s went out as expected, but the Boot Device l.e.d. remained red. The front panel SATA HDD activity light came on briefly twice during the startup but failed to actually load as it did before and there was no HDD activity after the initial two times. I got no "ASUS" flash screen on my monitor, I got nothing.......

Since I could not get the bios to post on startup. I tried to reset/clear the cmos by removing the power cord, removing the battery and jumping the pins on the Clear RTC for several minutes. After I put everything back to normal I tried to startup again, but again - nothing.... In an attempt to disprove the definition of insanity, I tried this several times over adding or removing disks........ I was not able to get the bios to post nor was I able to successfully redefine insanity.

I can understand the SSD and now GPT HDD not booting in what now should be "Legacy" mode after the cmos reset(s) however, I do not understand why an MBR formatted Win10 USB bootable disk does not work.

The bios post failure happens no matter if I have the SATA HDD connected or disconnected, SSD on PCIe card in or out, Win10 bootable USB in or out or any combination of the above. I thought if the bios was reset to "Factory" it would post and give me an error if it was unable to find a boot device. I am at a loss as to explain why the bios fails to even attempt to post.....

Did I go to extraordinary measures to create a "BRICK"???? or is it recoverable somehow... Any insight would be helpful.

[UPDATE]

Well it looks like I may not be going insane after all. After removing the power & battery & installing the cmos reset jumper and leaving it in this condition for about 2 hours, the bios posted properly and I was able to change the boot to EUFI Compatible ROM with the SSD as the first boot option. Win10 successfully booted from the SSD as expected with all drives connected to the MB. The HDD showed up in the bios but did not show up in Win10 Disk Manager. I restarted the computer and changed the SATA controller setting for the HDD to "Hot Plug" in the bios and rebooted. This time Disk Manager saw the HDD just fine. Coincidence?

I am still going to attempt to "restore" the old OS to the new SSD, however, I will be using Macrium Reflect to image the old OS partition and try to restore it to the new SSD. I have created a Win10 backup image of the currently working SSD OS in case I run afoul again. Fingers crossed.......

[UPDATE]

OS image copied with no problems - Back to previous Win10 environment with all programs, desktop, etc, but disk access went from ~200 Mb/sec read/write on HDD to around 1800 Mb/sec with SSD on PCIe card. I am sure that the less than published read/write speeds are due to my I5 2nd gen CPU vs the more advanced 8th-9th gen I7 or I9 currently in use. But overall very pleased with results.