[QUOTE=Slartybart;679605]Depends on the goal - are you determined to have GPT?
Option 1Set firmware to legacy (or cms)
Disable Secure boot
This will let you run the system on the existing MBR drive
Option 2Run
Belarc to get your software audit (including keys)
Make sure your Win10 install media is on a Fat32 formatted USB with - or - on a DVD
Leave firmware as UEFI
Disable Secure boot
Clean a spare drive (no partitions, not initialized MBR or GPT) - diskpart clean
Install Win10 - it will initialize the drive GPT and create the correct partition schema.
Install your favorite apps - but get the latest from the vendor site (too many bugs caused by old software)
Read through this tutorial and ask any other questions:
Windows 10 - Clean Install[
In reply: A short story. I'm typing on my portable tonight. Had to take my PC to an experienced service shop
today. My problem was because attempts to refresh Win10 kept resulting in failure and notices that I had "unsupported layout for UEFI firmware". That led to the attempts to change MBR to GPT because I read advice that UEFI worked properly only with GPT. (It is now becoming clear that is not true.)
I finally started moving copies of the C: partition to another location so I could change the boot drive too GPT (it had to be bare, so I learned). At that point chaos ensued in the BIOS. I wound up with 14 instances listed in the BIOS that all said "Windows Boot Manager"
Trying to correct that (!) I reached the point where all the machine would show was the ASUS logo. That was the point at which I unplugged everything and hauled it downtown. The tech is older and quite experienced with "enthusiasts" like me. Smiling, he said: "You guys tend to overthink everything." I had to agree. ....But I'll probably do it again someday. Thanks to all of you for your advice of any kind. I genuinely appreciate this group!