If you have collected a library of media files and they are in a certain format, chances are that all those same formats won't work on a tv or dvd player. (Most media players, like the WDTV, can play nearly ALL formats).
A common occurrence is if your tv plays only DivX files, and you have hundreds of gigs of MKV files.
The solution is to convert your media files into a format that your TV understands. HSTi recommends a very good free product called XVID4PSP for windows. If you use a Mac or Linux, you can use a program called HandBrake.
http://www.videohelp.com/tools/XviD4PSP
for the mac or linux -
http://handbrake.fr/ (works in windows too)