Yes. This is a BUG. As Adobe hasn't enabled a way to limit the Video cache size.
Why did LR create such huge media cache files when I copied mp4 video files that have NOTHING to do with LR?
Did you import them into LR. I recently accidentally clicked on a folder that has old Batman TV videos in it while I was in the LR import dialog and after reading your original post went to the folder you listed and found 35+ GBs of files in that cache folder. Seriously I just Clicked on that folder then off of it not 5 seconds later and LR created 35+GBs of cache files.
What version of LR are you using? If it is still the 2015.2.x/6.2.x version and you get that lousy start screen to the import window that SEARCHES all your drives looking for images to import. That could be the cause.
I am using LR CC.
I just opened the files on the Creative Live website, selected Save As and downloaded to a Photography Training folder on the secondary SSD I have. It NEVER involved LR from my end as far as I know. Certainly nothing overt on my end.
I did just go to LR Preferences and clicked on the purge button for the Camera Raw and Video cache (supposedly limited to 1 and 3 GB, respectively) and lo-and-behold, ALL 104 GB in the offending Media Cache File was erased. And happily, the mp4 videos STILL work.
So, looks like the acute problem is solved BUT I would still like to understand why LR felt obliged to create 104GB of files for downloaded training that had nothing to do with LR.
I am confused
LR tells a separate process to scan for and create cache files from each video file in folders you click on to navigate where you want to import from, or maybe, now, from drives you import from. This process, once started, it independent of LR, so shutting off LR has no effect. Theoretically, if LR were to import those files, then it wouldn't need to build the cache, because the cache is already built for them during the prescan, but because the cache as built by the independent process, it doesn't know if LR is actually importing videos from the folders you've clicked on getting to the folder you're importing from, much too many videos are scanned and cached.
I was thinking that Adobe changed something in how LR 6.3/2015.3 works so it wouldn't do this scan so imprecisely but I haven't verified what this latest update still does, so I'm not sure if it's been fixed, or not.
Verify that the most recent LR 6.3/2015.3 update is still causing a problem and that your cache hasn't been filled up with caching from previous versions of LR before you go renaming the exe.
The dynamiclinkmediaserver.exe is the name of the process, and using Task Manager you can find where this exe files resides on the hard-disk and rename it if you want to stop the behavior from occurring, at least until the next Adobe product is installed, which will likely put it back.