Originally Posted by gtrscort
Isn't it taking the information from the hard drive on PC or laptop? So why wouldn't it work for the hard drive?
Sort of, but you're kind of hitting a semantic barrier. USB is a bidirectional
connection for transporting generic
data. On the other hand HDMI is (essentially) a unidirectional
connection for transporting raw video
Now the term "adapter" has the connotation of meaning of (in this context) converting one type of video data to another. So there's the issue, USB doesn't necessarily transport video, so there's nothing to adapt.
To put it another way: A USB video adapter (USB to HDMI for a PC) doesn't "take" anything, it requires something to "push" video data over USB to it, so that it can convert that video data to HDMI format. In addition to that it requires something on the PC to first select which video file to play, and second decode the video file (which is compressed) into raw video and pipe it out over USB.
The reason it won't work with a hard drive is because a hard drive doesn't have the capability of either selecting the video or converting the compressed video to raw video.
What you need then is a "device" that can select, read, and decode the video files on a hard drive and output the resulting raw video over HDMI. There are many such devices but the terminology is a "media player", which can either be a dedicated box (WD TV Live, Popcorn Hour, even some Blu-ray players - Dune/Oppo/etc) or some software on a PC.