If the DVDs in question are commercially available pre-recorded DVDs (just to distinguish from home movies), more than likely copy protection is what is keeping them from getting onto your player. If you can overcome that obstacle, then you should be able to get them on. There is software available to help with that, but I can't go any further on the subject because it is illegal to break copy protection on DVDs in the United States. Not to say that it isn't done. Just that further discussion on the specifics would violate the TOS of the forums and cause this thread to be locked.
I just bought an Archos 605 with the cinema plugin. I want to use it to take movies with me on long trips without having to carry a bunch of DVDs around. It's my understanding that the cinema plugin supports the .vob file type, but when I copy .vob files (or even the whole VIDEO_TS) folder from one of my DVDs to the Archos and select one of the .vob files to play, my player just hangs. I did some web searches that indicated that files larger than 2GB would be a problem, but I've checked, and the largest of my .vob files is 1GB. Is there some intermediate step or conversion that I am neglecting?

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