Files wont open

WD Legacy Products My Book Live
1

how come some files wont open saying invalid file, when opened from my normal hard drive they open fine. 

These are movies. the only difference i can see is that on the file name movies that open do not have .mp4 on the end and movies with .mp4 will not. 

 Exactlty the same file will open and play from my external hard drive. doesnt figure.

2

Hello, from where are your trying to play the files and what media server are you using, DLNA or Twonky?