So this question pertains primarily to Grub, moderators move this thread if there is a better place for it. (I didn't see one, but not sure what to look for, I'm a Linux noob.)

So I have, admittedly, an odd system configuration, and I'm trying to get Windows 10 Pro release 2004 and Linux Mint 19.3 to dual boot. It was working once, with Win release 1903 and Mint 19.1 until I trashed Windows printing system beyond repair and decided to start over. When it was working before, I just did normal installs but it's not working this time.
My system consists of an HP Z600 workstation, dual Xeons, 24G RAM -- and an NVMe drive (and a couple HDDs). Now the Z600 is kinda old and uses a legacy BIOS which doesn't understand NVMe drives and certainly won't boot from one. To get around that, I add UEFI extensions on top of the BIOS using Duet-Refind which boots from a USB flash drive, adds the UEFI extensions, loads the NvmExpressDxe-64.efi driver, maps the drives, and then runs Grub, which in turn allows me to choose Windows or Linux. Or at least that's how it used to work...
Since the reinstall grub flashes an error waaaaayyy to quick to see or even try to get a photo of, and then puts me in the Grub shell. I researched the commands of the Grub shell but didn't see anything that would allow me to edit the grub config file from there (apparently it has to be done with the Grub-Config program in Linux?) and likely wouldn't know what to change if I could.
I can quit Grub and get the UEFI shell instead, and from there I can run \EFI\Microsoft\Boot\bootmgfw.efi (from the FAT32 EFI boot partition at the start of the NVMe drive) and start Windows. But I can't get Linux Mint to start, I've tried \EFI\ubuntu\shimx64.efi and fwupx64.efi and grubx64.efi. grubx64.efi of course starts grub again, I get the error flashing by too quick to see, and get the grub shell. shimx64.efi (that's the one that should start Linux Mint, right?) causes the screen to go black, and after about 70 seconds, reboot.
It appeared that Mint was properly installed, the installer didn't crash until it got to the very end where it gave an error about not being able to configure grub. Best as I can tell, all the partitions and files are setup/copied but I can't get it to start. I have to boot from my 'live' usb version, and running grub-config from there wants to edit the one on the live disk, not on my NVMe or Duet boot drive.

Since its an odd system, and not likely that anyone here is familiar with Duet, I don't expect to get a full-blown solution from anyone, but can you point me to possible articles or searches that may get me going? Other than 'grub' and 'duet' are there other keywords I should search for? I'm very good with (but also very pissed of at) Windows, but don't know much about Linux (yet).

At least your not getting a grub-rescue> prompt. Here is a link that explains working from the grub shell. https://www.linux.com/training-tutor...-grub-2-linux/

A few things it doesn't mention are:
1. will clue you in on where the grub bootloader is looking for the grub.cfg

2. If /boot/grub/grug.cfg exist, but can't find it or is in a different location, once you find it searching with ls from the grub prompt, you can run this command to pull up your grub menu, ? will be the drive number the configfile is on counting from 0, the # will be the partition number counting from 1.

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And I thought I had some whacky systems ...

Don't bother with a photo - get a video running then boot it. Go through it frame by frame - might take a couple of goes to get something readable. I use a gopro on a small tripod, but a phone should also work. I know naught of Duet.

Thanks guys! I will look at the grub tutorial you pointed me to, and I also considered the video trick but haven't tried it yet (suspect it will not yield useful info but will try anyway). And yes, I have backups of all my personal files in several places. The point of all this was to get fresh installs of the OS's.
Here's some more info:
1. Apparently is was NOT completing the install as I believed. When trying to install grub I get this error: "The grub-efi-amd64-signed package failed to install into /target/. Without the GRUB boot loader, the installed system will not boot.", this was happening when there was (according to the progress bar) ~20% left.
2. I googled that error, found lots of articles and forum posts with possible causes and remedies. None of them really sounded like they pertained to my problem, but nonetheless I'm going thru most of them starting with the least unlikely.
3. As mentioned above, I had it working with earlier releases of Windows and Mint, without doing anything special. So, still having the ISO's on a network drive, I wiped my NVMe and started over again with those. Guess what? Can't get it to work this time. Trying to remember what I did the first time that was different.
4. If I'm reading correctly (?) several installation guides seem to suggest that if you can turn of EUFI in BIOS, you should, and let Mint use Legacy Boot Mode to install. But if I turn of EUFI (don't boot Duet) then the NVMe drive is inaccessible and unbootable. Tried it anyway-- Booted directly from Mint Live disk and didn't boot Duet first. Well Live Disk was able to access the NVMe (without having started the Duet EUFI) and this time it did complete the install without errors. It did not add/change/update grub on the efi partition, and after completing the install and rebooting it would not of course boot from the NVMe 'cause it couldn't see it, and then after reinstalling the Duet boot disk and running bootx64, grubx64, shimx64, etc. from efi partition all fail to successfully start grub or mint.

At this point I would be quite content to get the machine to boot only Mint, and I could run Windows or win Programs when I need them (less and less) in Virtual Box or Wine.

I was wondering if perhaps there was a way to get the machine to legacy boot from the SATA hard drive and load just a minimal amount of stuff for the kernel, including drivers to read the NVMe, then switch over and finish booting and running from the NVMe (which is like 7-10+ times faster than a SATA SSD). [Windows of course, has no such option.] Any pointers, links, advice on how I might be able to accomplish that with Mint or even regular Ubuntu?

A bit more googling should have found you https://delightlylinux.wordpress.com...en-using-nvme/ or similar. Have fun!

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OK, solved, I got it working! Thanks again to ColorPurple, the article you linked to had just the info I needed.
FYI: The way I got it working was-- Since the 'Live' disk was able to see the NVMe drive (without the EUFI) I booted without the EUFI into 'Legacy' mode and installed Mint that way, and was able to get all the way thru the installer. But it wouldn't boot because the legacy BIOS won't see or boot from the NVMe. So next I booted again with EUFI, and following the first part of the article suggested by ColorPurple, I was able to manually start it from the grub shell. Alas, I could not get the second part of the article to work, where you save the new grub config. No matter what I used as the drive to save it to, I got one error or another (depending on which drive).
Next I booted with EUFI and to the Live disk, and ran the install again. As before, it crashed at about 80% while trying to install grub. But whatever was left in that last 20% had been done successfully from the first (Legacy) install, so it was still bootable and even though the grub installer crashed, it nonetheless created a correct config. So now I could boot with EUFI, see the NVMe, and boot grub from it, which will now successfully start Mint or Windows.

No doubt I did this the convoluted hard way, but I'm still a noob and that's how I managed to pull it off.

Thanks to to MrMazda for his link. And I will admit I hadn't (yet) googled my second question regarding booting from SATA then switching to the NVMe. I was quite surprised to see an article so closely mimicking my own situation and how easy it is to do it. (I had a hunch it was). Setting up Duet to boot Windows from the NVMe was a complex pain in the butt and not for the non-technical or faint-of-heart.

I appreciate all of for you steering me to these articles!

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