Strange Movie Player Bug

I have had this strange Viewer Pane movie play bug for quite a long time - its annoying! I am running Windows 7 x64 with the latest version of Directory Opus. I have numerous movies stored on an extra hard disk in a folder on D:\ drive; .mov, .mp4, .avi, etc. I have no additional codec packs installed. The problem...

While the videos are located on my extra hard disk on D:\ I am not able to watch them through Viewer Pane yet if I were to copy those same videos to my OS C:\ drive and then try to watch them they play fine. If I were to double click on those videos whilst they are located on D:\ they play fine in Windows Media Player. Enabling Windows Media Player in Directory Opus' settings, ActiveX Preview, helps a little bit as it is then capable of playing some of those videos in Viewer Pane - but not all. Yet if I were to copy any of those same files over to my OS drive and play them in Viewer Pane there are no problems. The viewer plugin, "Movie", is enabled and the file extensions are configured.

I have checked the drive and folder(s) security settings and everything is okay as far as I can tell. I am stumped as to what the problems is. Any ideas?

Opus's main Movie plugin will use 64-bit codecs/splitters, while WMP usually uses the 32-bit ones, so a difference in what's installed between the two types may account for the different behaviour.

But it's odd that anything would care about which local drive the files were on, unless one drive is much faster than the other, or there's an intermittent drop-out on one of the drives (possible with USB drives sometimes, or motherboard SATA/IDE driver bugs, but pretty rare outside of the issues people are seeing with Windows 8.1's new USB3.0 drivers), or an antivirus tool is treating the drives differently, or some other weird problem.

Are the two drives using the same filesystem? (NTFS for C:\ presumably.)