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I had Windows 10 BIOS boot on one disk and Ubuntu BIOS boot on another disk. There was no problem booting either from grub.

I took advice and reinstalled Windows 10 with UEFI boot, as it refused to upgrade.

Now I can no longer boot Windows from grub on the BIOS disk. I would prefer to keep the Windows bootloader on the Windows disk

I tried chainloading Windows from an Ubuntu UEFI install on flash drive and that did not work either.

I have tried everything I could find on the Ask pages, nothing has worked.

Is it possible to chainload Windows 1909 release using grub?

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    Your Windows install is UEFI, so cannot be booted from a BIOS grub install..You should be able to boot the disks separately, some machines more conveniently than others (some allow you to select a preference for first boot). Chainloading UEFI windows works, it uses the full path to the efi bootloader, not -1 like BIOS chainloading. Check how your installer boots (machine preference).
    – ubfan1
    Dec 18, 2019 at 5:50
  • @ubfan1: The computer is Gigabyte GB BXi7 5775, no problem booting either disk using F12, but was more convenient when one grub suited all. If I made the Ubuntu install UEFI would that work? as I said above, a Full UEFI install to flash drive does not boot the Windows either. Dec 18, 2019 at 6:04
  • Ubuntu 18.04 in both UEFI and BIOS mode boots Windows in the same mode on my machines, The "full install to flash" may be a different problem -- check that is really is a UEFI install. You had to change partition table from MSDOS to GPT on the first disk, so new UUIDs needed in the grub boot on disk2, but it wouldn't even see a UEFI install so I don't know what state grub is on disk 2.
    – ubfan1
    Dec 18, 2019 at 17:08
  • I have tried 18.04 and 19.10, The Full flash drive was made with a mkusb base so it boots either BIOS or UEFI on the Gigabyte. it has GPT partition table. The UUID's match. full path to efi bootloader is specified. Wonder if the problem might be release 1909? Dec 19, 2019 at 11:39
  • Wondering why the downvote? It would improve Ask if comments were required with downvotes. Dec 19, 2019 at 11:48

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Booting Ubuntu and Windows using grub with UEFI

I gave another try creating a new Ubuntu disk using UEFI and installed it next to the Windows disk. I then ran sudo update grub and now both the Linux disk and the Windows disk now boot from grub.

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