Try to unmount it, then remount it again.
$ sudo umount /mnt
$ sudo mount -t btrfs /dev/nvme0n1p2 /mnt
Not yet. I would want to wait with any writing changes to the device until I have an idea what is actually wrong.
Edit: And the suggestion for that command comes from a page that doesnât describe my problem exactly, Iâm hesitant to try to write anything based on that.
It looks like the issue:
I found this as well, but I think the error message is pretty generic, and my system doesnât have LVM. The thread describes the duplication of a BTRFS volume, then not being able to boot it, and the cause is a duplicated ID, which is fixed with BTRFS tools. Sadly, this is not my situation.
Have you tried the LTS kernel?
I would not know how to do that in the current situation. Reboot the live media with a different kernel?
Edit: If you just want to try an older kernel, this is an older kernel:
Linux EndeavourOS 5.15.8-arch1-1 #1 SMP PREEMPT Tue, 14 Dec 2021 12:28:02 +0000 x86_64 GNU/Linux
That is a good point. You probably canât. Although the current live media is older now so the chance of finding a bug in btrfs is pretty low.
You could try booting a newer version of the ISO which would have a different version of the kernel.
If that doesnât work, I would send a message with the details on the btrfs mailing list. That is where the btrfs experts live.
How did you duplicate the volume in the same file system?
I would test it in VM to see if it reproduces the problem.
I have no duplicated file system. I was describing what the thread you linked described, and that it is different from my problem.
So, an update. I went to an IT vendor/repair shop on friday, bought a new m.2 SSD, let them put it into my computer, removed the 2TB drive that was causing trouble and put that into an external USB drive.
I could without any problems reinstall my system to the new drive. The old, now external drive, when trying to mount it, shows exactly the same problem using the external USB drive/adapter. Iâve also tried WinBTRFS, mounting the now external drive using the Windows driver that shares no code with the linux kernel driver (in case its a software bug), but I had no success there either.
So as it currently stands I would say that the original drive somehow has a fault that prevents mounting, but not reading the drive sector by sector.
And as an update to my post to the BTRFS mailing list, nobody replied to that.
What is the 2TB drive make and model. Is there info on it that you need or can it can be wiped? Does the system see the drive when plugged in? You just canât mount it in order to access it?
It looks like there was an issue with his filesystem.
However, this post was from two months agoâŚ
I just wondered if the drive was usable? Because if it couldnât be mounted was he able to just clean the drive and create a new partition again to make it usable.
I have the drive still in the external (USB) controller. I have the hope that I can access it once again when maybe BTRFS tools can find out what the problem is/was.
Does it have info on it you need or you just canât mount it to use it?
Thereâs info on there Iâd like to get back. I use decentralized software like Freetube and Kodi and my last backups from their databases was a bit ago, so I lost the status of watched shows/videos, as well as the last bookmark backup for Firefox and the settings of all the Firefox extensions. I can rebuild almost everything, but could save time when I would regain access to the drive.
Did you try mounting with âmount -o recovery,roâ.
If that doesnât work, try âbtrfs restore /dev/sdaX /mnt/restoreâ.
Based on the log âbtrfs rescue zero-logâ should help, but it does perform changes to the filesystem, so I wouldnât try that without backup.
(POLL: Which file system in 2022. ext4, btrfs, xfs, zfs, f2fs (choose 1) - #210 by mnrvovrfc)
I confess I was harsh when I made this post and others in that same thread, but Iâm willing to have my mind changed. I hope the situation described in this thread is resolved soon. 
However, âbtrfsâ so far would have been an absolute no-go on my other HP laptop which has the same type of SSD. Also since itâs only 64GB, I doubt it would be good for anything if I wanted more than 2 or 3 snapshotsâŚ
I had Fedora 37 for a short time on that SSD, but took it off for Spiral KDE. Did it without thinking that it could have given me a problem not easy to resolve. I donât care about a rolling-release thing on that other computer since I almost donât go to it anymore.
I have so far consciously not done any writes to the device, as I still have hope that the situation gets better. Doing a backup is tricky, as I do not have 2TB around to write the dump to. All mount tries I did read-only (-o ro), but I could add the recovery option to try it out.