Wei

One saving that is real and much less fiddly: shader caches and the compatibility layer's downloads are not the big number. The big number is the game files, and for the handful of genuinely enormous titles most people own, they are almost all Windows-only.

So the pragmatic version of what you are asking is: share the drive for those five games, keep everything else separate, and stop optimising. That gets you most of the space back for none of the fragility.

Before you delete anything: if this really was an experiment, take one snapshot of it and put a retention date in your calendar for a month out.

A snapshot of an idle dataset costs nothing in space, and "I deleted the experiment and then needed one file from it" is a specific kind of annoying that is entirely avoidable for zero effort.

Nothing is lost. The operating system is intact and so, almost certainly, are its boot files — what you have lost is the firmware boot entry, which is a small record in the motherboard's NVRAM that says "there is a bootloader at this path on this disk". Recreating it takes about ten minutes from a live USB.

What removes it. In rough order of likelihood:

• A firmware update. Updating the BIOS or UEFI firmware very often clears NVRAM, which wipes every custom boot entry. Windows recreates its own entry automatically on the next boot; Linux does not, because nothing is running to do it. This is by far the most common cause of "it worked for a year and then one day it did not".
• A CMOS reset or a flat backup battery. Same effect, same reason.
• A Windows feature update. Less common than the internet claims, but it does happen, and Windows will happily set itself first in the boot order even when it does not delete anything.
• Fast Startup. This one does not delete the entry, but it produces a machine that appears to boot straight to Windows regardless of what you choose, because Windows was never actually shut down — it hibernated. Rule it out early because it is the cheapest to check.

Getting it back.

1. Turn off Fast Startup in Windows first, before anything else. If that was the cause you are done. If it was not, you still want it off, because with it on the Windows partition is left in a hibernated state and anything else touching the disk is risky.
2. Boot a live USB of the same distribution family. You may need to disable Secure Boot temporarily to do it — note whether you did, because you will want to consider turning it back on afterwards.
3. Mount your existing root partition, mount the EFI system partition at its usual place inside that, bind-mount the pseudo-filesystems, and chroot in.
4. Reinstall the bootloader to the EFI system partition and regenerate its configuration.
5. Reboot into the firmware and confirm the entry now exists, and that it is above Windows in the boot order.

The step people get wrong is number three, specifically which EFI system partition. On a machine that has had more than one install, or that shipped with a vendor recovery partition, there can be more than one, and installing the bootloader into the wrong one produces a bootloader nothing ever reads. Identify it by looking for the small FAT32 partition flagged as the ESP that already contains the Windows boot files — the two operating systems share one ESP on a normal dual-boot setup, and that shared one is the right target.

If the entry keeps disappearing after every reboot, that is a different and rarer fault: some firmware aggressively prunes entries it does not recognise. The workaround is to place the bootloader at the default fallback path on the ESP, which the firmware will boot without needing an entry at all.

Quick way to confirm the diagnosis before you touch any settings, from the live environment you already have booted: open a terminal and list the block devices, then list the NVMe devices specifically.

If the disk is absent from both, it is invisible at the kernel level and it is a controller or firmware problem — the installer is innocent. If the kernel can see it and only the installer cannot, that is a completely different and much rarer situation involving the partition table.

Thirty seconds, and it tells you whether the sequence above is worth doing.