I have 30 or so of our movies ripped and backed up onto my internal drive. Starting to run out of space I bought a 320gig external usb drive, and copied the movies onto it. Vista won't actually play any of them. Using mediaportal I just get a blank screen (the drive is working and there are no error messages). Using WMP it errors out complaining the drive region is wrong (yeah, umm right).
Interestingly on a fresh install of vista (no updates applied) I dont have a problem. I dont know if this is a bug, whether an update is borking something, or whether it's DRM related.
If I copy the movies back to the internal drive, they play fine.

Anyway the solution is to make a hard link to the external drive eg

>cmd<
mklink /J mymovies E:/movies

Where E:/movies is the external drive and directory I have the movies stored on, and mymovies is the local folder I want the drive linked to.

Now when I point mediaportal to the linked directory all movies play fine, and WMP plays without the strange 'region error'.

Hope this saves someone with the same problem a few hours of hair-pulling.