I finally got some time to work on this (sorry for the delay) but without success.

I used the trick of holding shift while clicking the button to restart on the sign in screen, and it took me to the list of options, so I chose to use a command prompt, which gave me an error code 80070070.

But one of the other options was to reinstall Windows to the default settings (I'm pretty sure I have a hidden partition that has the ability to do that, and I think that's what it's referring to there).

I'd like to avoid this is possible, but I may need to use it if all else fails. It's not that big of a deal, since I can back up everything that I need (and have mostly done so already). Of course, it would probably wipe out my Linux partitions as well, but again, that's all backed up, so it wouldn't be too difficult to get everything back to how it is now.

However, after I got the error code for trying to use the command prompt, I tried restarting the same way again, and the next time, my "list" of options only included to shut down the computer, and nothing else!

So without being able to use the command prompt, I can't create a user or set its permissions, and I can't even get the Windows product key for use if I need to reinstall (and I can't automatically authenticate online because I keep Windows 10 offline at all times).

It seems like restoring from the hidden partition might not require the product key (I'm not sure whether it would), but even if it doesn't, that doesn't seem to be an option anymore! It might work if I try again, but I don't know, because after that I just came back into Linux. And even if it does give me the option again, I'm not absolutely sure that it won't require a key which I don't have!

Anyway, I looked up error code 80070070, and this link says that it has to do with having too little space available on the hard drive during a Windows update. I don't get updates because Windows is offline. My Windows partition is somewhat low on space (I've installed too many games I guess) but there are several gigabytes remaining, and I've never had this problem before, even when running out of space.

Also, this post suggested that it was caused by or had something to do with OneDrive, which I don't use at all.

I found a couple of other pages on the Microsoft forum that said pretty much the same thing, but other than that my search didn't tell me anything else.

So what can I do now? So far I still haven't changed anything at all.

You could have already reinstalled windows by now. Or...don't and just keep using Linux. I don't think solving this esoteric problem is worth it when you have access to the drive from Linux to backup anything important & start clean.

First, merely rebooting/restarting and doing a complete power cycle are different things, and may reveal different options (same goes for BIOS/EFI options). Try turning completely off and then start over and the recovery mode will automatically kick in after a few times:
https://support.microsoft.com/en-us/windows/start-your-pc-in-safe-mode-in-windows-92c27cff-db89-8644-1ce4-b3e5e56fe234#WindowsVersion=Windows_10 And, the error message might indicate a corruption in the system files, and that's where SFC and DISM comes in to correct/restore. https://www.youtube.com/watch?v=W7vOo9NW-fo

These tools are simple but can fix A LOT of problems for games. I run them at least 5-6 times a year.

There is a limited "chroot" method on Windows... but you need to boot up WinPE or some installation media for Windows if the regular method chronically fails: https://www.wintips.org/how-to-run-sfc-offline-system-file-checker-tool/

Some recovery drives, if it's OEM or pre-built, have the option to use the Windows Recovery Environment (advanced recovery).

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Gotta say, GRUB usually behaves pretty well. Been a long time since I've had much problem with it. Had some problems back when Sabayon was active but that's years ago. Even my mother regularly update Mint and there have been zero problems there, though I'd wish the developers set a limit on how many kernels there should be by default because after some time the boot partition was completely full, and the GRUB boot list was pretty long...

EDIT:
https://www.hirensbootcd.org/
https://www.aioboot.com/en/hirens-bootcd-pe/
How to create one yourself: https://www.tenforums.com/software-apps/182581-win10xpe-build-your-own-rescue-media-2-a.html

That's the thing; sometimes it's faster to just reformat and start over, yes, but consider editing options and stuff like it was before it crashes takes some time unless one have a complete image setup. At the same time, one does learn much by trying to fix it yourself, that's when one learn pretty quick where one might save time.

Like I wrote - in many cases I've seen people would rather take the hassle of reinstalling and doing endless re-configuring rather than just running two lines of code (like SFC and DSIM, three if CHKDSK is included) which could've just fixed it in a minute or two. Seen it plenty of times on Steams forums... all because they' don't know better, are unsure, or just fears the dreaded (haunted?) COMMAND LINE. :D

eol

Well, on this issue in general, as in one OS doing something to the other, QEMU/KVM PCI Pass-through is always an advanced option to separate the systems entirely and to allow to run 3 OSes at once with almost the same performance as 1 OS (graphics will take a 5% penalty, sadly). Bear in mind, doing so is not for all motherboards, this is RAM-hungry (for obvious reasons of the 3), might require 2+ GPUs, plus knowing the command line. Is it for everyone? No. Should it be mentioned? Yes, it doesn't get mentioned enough, not all know this is technically possible.

Same, that's why I am still on 7, instead of the headache. As long as it runs what I want for games, I prefer stability/predictability. Linux is of course set for the Internet (security reasons).

Not being afraid of the command line is a savoir.

For example, I once accidentally nuked the GPT table for an EXT4 drive, `testdisk` helped to recover all the data just like was before, easily. Knowing basic Python/Shell scripting makes any data management easy on no matter how large the scale is or the process is "personalized," while doing specific tasks on Windows would be either impossible or very convoluted at best. The limit on Windows to me is "with what can I do it, where can I find software that can do it, is the software safe?" but on Linux is "how should I do it?."

Well, I really don't think GRUB is the problem, sanscript, because of the fact that Windows can boot at all, but it does make me wonder, why would Windows be having a problem at all, after installing Linux, considering that I changed absolutely nothing on the Windows partition? But thanks for the advice.

EverNightX, it seems like some tiny thing must have changed, since all of my data is still there, and after all, I didn't make any alterations, so if I could just change that tiny thing back to how it was then I wouldn't have to reinstall the OS. And it does take a lot of work to get everything back to how it was, especially since depending on how I may have to do it, it might wipe out Linux too, so I'd have to start over there, and I have a lot of customization that I'd need to set for it, and some for Windows as well.

anitmetee, I'm not sure what you're talking about exactly, but I don't think it would be necessary for what I'm doing. But I'm not afraid of the command line. I'm a programmer, but I'm just not very familiar with DOS, because I haven't really had to use it much, and not in many years. All I do in Windows is play games.

I may look into some of the stuff that sanscript said about the WinPE thing, but I think what I may try first (once I have some time to do it, and now that I've finally finished backing up my files) will be to install Windows on a new partition using the DVD, see how that works, see whether it expects some kind of authentication, and if so, whether I have a way to get it (by the way, is there some way that I can get my key from the other installation which is already there, when I'm not booted to that?). Then if that goes well I may either just leave it like that and use the other one for storage, or reformat it and try to reinstall on that one. I don't know yet..

Fair enough, I agree QEMU will be overkill.

The key, if I am not wrong, typically Windows 10 writes it somewhere in UEFI, so the newly installed system should autocatch it fine.

Maybe I'm reading this wrong - but I never once thought it was GRUB...I even advocated it was way past the initial boot steps.

OK. But my prediction is you'll be no further to solving your problem 1 week or even 3 weeks from now and you'll end up reinstalling anyway. Which is why I suggest ripping the band-aid off and be done with it. You know how to install Windows. You don't know how to fix this problem.

I'll say no more.

anitmetee, what's QEMU?

sanscript, yeah you probably said that, but different people said different things and it's possible I got confused and misremembered something, or else you were just talking about GRUB, and I was pointing out that I didn't think it was the problem. I don't know, and it gets a bit confusing after a while, trying different things, failing, reading and writing posts, repeating the whole process, etc.

EverNightX, I realize that, but I also know the danger if I happen to reinstall Windows and then realize that I don't have a key to authenticate it, so I'm trying to be cautious.

Darvond, I don't use it in a serious capacity either, but I use it for playing games, because about 70% of the games on this website (and it seems 90% of the games I own from it) aren't compatible with Linux.

EDIT: BTW, I'm not familiar with that table or index listed under your name; what does it do and why would you drop it? Aren't indices just for improving search efficiency?

Yanno Dosbox? Imagine that, but at a much deeper level. Basically a virtual machine.

As for the matter of incompatibility is that figure with, or without Wine involved? Cause Wine opens many doors, especially after Valve got involved.

This will explain my title. I wanted to find a way to break the forum in a way that'd force them to improve it, but they've sadly sanitized their inputs.

By the way, if the only purpose of your Windows installation is gaming, I will concur with some others here and propose you ditch it completely for Linux. There's only a smattering of games that do not work on Linux these days and something tells me you're not the kind of guy who plays those multiplayer kernel-level anti-cheat games.

Darvond, yes I'm familiar with both DosBox and VMs, so I understand what you mean now. I don't use Wine or any of that though, and I'm just running Windows 10 directly, and then reboot to use Linux. So you're using SQL to inject code into a website forum? Well, alright.

rojimboo, what do you mean? There are a TON of games that only work on Windows, or at least are only available for Windows, on GOG, anyway. Just go to the store page and check the filter to only display the Windows ones, and it will display ALL of them, but if you check to only display Linux then it shows only about 30% or so. You can tell by the page count. And I happen to know that most of the games that I have are only compatible with Windows, and very few are compatible with Linux (and some of them are even a pain to get to work right).

Curiously QEmu has a module you can insert into the kernel to have the code run nearly as fast as native, similar to what VirtualBox does.