Not All Movies Play on External USB Drive

I have a 1st gen ATV and have successfully connected a 2TB WD MyBook as the primary drive.  I have filled about half the drive (about 1 TB) with movies, TV shows and music.  This setup has served me well for about a year and until now, have had no problems playing/watching any files until the past few weeks.

Now I have a problem where certain (old/previously watched) movies play without any problem but when I try to play unwatched movies they either (1) freeze up the ATV forcing me to reset by unplugging the ATV or (2) playing the unwatched movie will somehow force a reboot of the ATV.  All files play fine from within iTunes.  Thinking this was a software issue, I restored my ATV to factory specs, downloaded an unwatched movie directly to my ATV's original drive and it played fine.  I then re-installed Firecore and re-connected my 2TB drive.  Again, I can see all movies, TV shows and music files, but I'm having the same problem playing unwatched files.  I even tried playing the files from within iTunes (to remove the unwatched blue dot) and tried to play them again from my ATV, but to no avail.  NOTE: prior to having these problems I went about 6-8 weeks where I was away on business and wasn't home, though my ATV (and external HD) was left on and connected.

The only work-arounds I can think of defeat the reason I purchased ATV - i.e., I can either stream from my computer or get rid of ATV and use just the 160GB drive on my ATV to watch unwatched movies (which for whatever reason still works fine).

Anyone know why I can play some (old) movies on my ATV, but not other (newer) movies?  Has my external USB drive gone bad (bad sectors?)?  I had disconnected my 2TB USB and connected it directly to my Mac and I was able to copy files to it without any problem.  Also, is there a recommended "best practice" in terms of using an external HD connected to the ATV - e.g., should I be turning off my HD when I'm not using it (especialy when I'm gone for 6 weeks)? Any help is much appreciated.