Download and run Seagate's diagnostic tool on the 2 TB drive:
(I have been using the bootable CD image for years ("legacy"), but it
looks like they finally have a bootable USB image.)
Use a camera and take pictures of the various screens. Transcribe them
into a reply to this list.
Dear David, hi guys/girls that are following this problem.
First, the delay in my response is due to the fact that I'm fighting a lot of parallel battles here, but I'm still trying to solve this problem.
In the next days I'll always try to reply.
I've done the bootable seagate HD test. It came up 100% ok, all tests. The HD is new, I wouldn't expect different, but it is always reassuring to do the real test and see the results.
To recap, I used the computer all day, since morning until late night. From time to time, say, about 3 times that day, I noticed a small 5 seconds long freeze. Sometimes, in other days, KDE wouldn't open plasmashell, but I did not need to reboot; just open tty1, and run systemctl restart sddm, and it would run ok. Not sure if this is related.
That night, I turned off the computer, no errors, nothing.
Next morning, first boot, it booted fast and I was faced with an empty desktop. Also empty /home. After e2fsck, all my files reappeared as inode numbers in lost+found. I was able to recover some of them, and I have backup for others. But I'm not still putting them on the hard disk.
I'm still waiting for possible causes or solutions, or just to know what have happened in the first place.
Is the "swap" partition something that could cause that if turned off by "swapoff"? Because I remember turning it off during that afternoon. But I used the computer a LOT until night, so, I don't think there was any link to the disappearance of ALL files. Also, when I used swapoff, it was almost without use anyway.
I read above from Abdullah, that it is very unlike to have such major FS fail, and it looks like it didn't "flush" the night I turned off. This is also my guess, it is the only thing that make sense. Maybe the inode table was in the memory and got corrupted. But it is strange to figure that, since the filesystem is EXT4, and it is very stable nowadays.
Someonelse asked if 2TB is ok for this laptop, Causey I believe, my inbox got deleted, but yes, this configuration accepts a 2TB; also, this would be a problem during the installation 3 months ago, and the consequences would be far different from the reported problem here.
Thanks all for inputs and lets reason what else. I may be able to create a virtualbox to test some theories in the future, to get to the bottom of it. The laptop is still in guarantee period, I need to know if there is something really wrong or if it was just an occasional, yet rare, event.
PS. Sorry for not copying Abdullah and Causey on the reply, my inbox got cleaned here.