Sure, first thing I did: click on "find in folder", it is there. Then click on "find in explorer", I can play it there with Windows tools.
I revisited the problem:
I originally had all source files and the catalog on an external drive with letter L.
Eventually, system 1 did connect to all videos there.
I then copied the complete content from this external drive L to the internal D drive of system 2.
I then assigned the letter L to this internal drive D from the system 2. Then I started LR, opened the catalog of the drive L (which was D before). The catalog did find its files ok, but as said, did not connect to some videos.
That was the problem I reported earlier.
Today I connected the external drive with its original letter L to the system 2 where I left the letter D for the internal drive which still contained the catalog. I launched the catalog from D which connected to the source files on the external drive L and found all files there. And.... Sloooowly it connected to all videos and played them!
Before it did that, LR spent 2 minutes re-shuffling folders. That was strange because for LR, the source files were always on an L drive. But LR must have recognized that the disk had changed.
And that seems to be the heart of the problem. I think I see more of this problen in other posts.
Could you explain what LR sees in a disk besides its assigned letter? And why it is not good to trick LR by changing drive letters?
Also, I still like to know where LR keeps its videocache and if one needs previews for videos.
Well, its not about the Lightroom. It is how Windows OS works.
If you have noticed and worked on Mac, it show all the external drive connected to it with name like Segate Or WD, In Windows whenever you connect a drive it assign letters to it and that's how your programs see it on Windows.
It happens on Windows machine when you have multiple external drive and you connect and reconnect multiple times. best way to correct it if the drive letter gets changed, then change it again via disk management on Windows.
Meanwhile I found the location of the Videocache:
C:\Users\strob\AppData\Roaming\Adobe\Common\Media Cache Files
"strob" is my username
It was pretty big and set to unlimited. Nevertheless, LR would not display Videos (again). I deleted all files there (took 2 minutes) and then it worked again.
It would still be good to know if previews are needed for Videos.
After deleting the Media cache file, if you don't loose anything from Lightroom like editing an all, so its fine.