No, reformating is only part, you need to set the disk type to msdos not gpt, which is an extra step. Hardware manufacturers expect systems to be UEFI and set to gpt type these days…
If you look at your output in post number 5, in your output the line with “Partition Table” you have a mixture of msdos and gpt, any disk you intend to boot from in your case sda and sdc need to be of the same type;
For legacy boot type msdos
For UEFI boot type gpt
Any other disks (data storage) can be of either type it doesn’t matter, however in saying that I prefer gpt since it protects the mbr area from being written to…
Legacy boot IMHO is old school now, if you plan to go UEFI then I would re-install both operating systems but use two efi partitions since you have multiple disks…
Perhaps advise your plan and see what info our forum users can provide you
And that’s why I would need to be physically at the computer to see what is on the screen at each step. Giving a sequence of commands for “gdisk” (as I did) can only be based on guessing what the response would be.
You could have kept a record and reported back in this thread on where you thought something was going wrong. But you have not done that.
… it still works flawlessly in the latest TW and in Leap 15.0 for most of us.
Which is why Neil suggests to you that you need to find a Linux person near you and get them to come in and take a look, see what is causing your unusual problem. It seems to me, reading this long thread, that is going to be the only solution to get you up and running.