Missing Files not Missing...

I have all my media on an external 4TB drive which is directly attached to my Mac Mini via USB 3.0 and which my MacBook accesses through Sharing on my home network. At the moment, both computers have WiFi turned off and are directly plugged into the router. Both the iTunes library on my Mac Mini and my MacBook are set to see this drive and all of its folders and files as the source for their media in the Advanced tab.

Previously, I had all my media on two external 2TB drives with this same set up, having used TuneSpan to allow both machines to find the files I put on the second drive. Previous to copying over everything from both drives to the new 4TB drive everything was fine with both machines seeing everything they were supposed to both before and after using TuneSpan.

I will note that the two machines are using different iTunes .itl library files, with the MacBook having less content in general but also a number of songs which are not included in the Mac Mini's .itl file.

On my Mac Mini which is running the most recent versions of Mt. Lion and iTunes, everything is good, all files are seen, no problem. On my MacBook which is running Leopard and iTunes 10.5.8, there are a number of movies which have exclamation points next to them. When I try to direct iTunes to locate the missing files, it sees the files just fine but the Open button is always greyed out so I cannot select the files; these same files are found and play fine on the Mac Mini and the file sizes are the same on both the old 2TB drive and the new 4TB drive. When I recopy the file from the old 2TB drive to replace the existing copy and then again direct iTunes on the MacBook to locate the file, it will now allow me to Open it even though it's the exact same file. When it asks me if I want to use this location to find the other missing files, it goes through some process and tells me this was not possible.

If I grab a bunch of the folders for the films that this is happening with at a time and recopy them over to the 4TB drive, the MacBook's iTunes continues to refuse to see the files. Only if I individually go through and recopy only the exact file it is refusing to use will it then agree to Open and Locate the file.

This is effecting about 65 movies which vary in size from 700MB to 3.5GB. They are films from every genre, some converted from VHS, others extracted from DVD, others re-encoded from .avi or .flv and some which originated as .mp4s.

The only other thing which seems worth noting is that the video files which the MacBook refuses to Open are depicted as music files with a music note on them despite them all either ending in .mp4 or .m4v and on the Mini they are shown as they actually are with the album art as the icon.

What is going on?! Why is it doing this?! How can I force the MacBook's iTunes to use the files it sees?!