The VCD and SVCD format for video disks was popular back in the day; it essentially let you create a video disk on a CD disk (not DVD) and play it back (usually) in a regular DVD player. Embracing what I thought was a pretty spiffy technology, I converted several videos from VHS tape into playable disks - this was before I had a DVD burner in my PC.
I dragged some of these VCD disks out with a view to converting them to video files that I could upload to my cloud account, as they contained some pretty special scenes of our kids when they were little. We don't have a DVD player connected to our TV anymore, so I reasoned just converting each VCD to a single video file made sense.
However, I ran into a major snag in that my Linux laptop system did not seem to want to do anything with these files; while I could see the file structure of the disk, I could not open the actual .dat video files, and I could not even copy/paste them from the CD.
I thought to my dismay that the CD media may be bad, but after I tried 3 or 4 different CDs and got the same result, I thought it would be unlikely that they had ALL failed (though not impossible). I went to our venerable Windows 7 home PC and was presently surprised - and relieved - to find that I could now open the VCD disk in VLC Player and run the conversion to .mp4 with no issues.
I could NOT open these in VLC on Linux, for whatever reason - I did not delve into it further, as this was a sort of one-off situation, and I already had a working alternative.
So if anyone else out there has some old VCD disk and need to covert them to a more convenient and portable format, VLC will do that for you - at least in Windows. Just open the CD, use the "SVCD/VCD" option (screenshot below, from the Linux version of VLC) and then use the "save/convert" feature to process the disk contents. Works like a charm.