Okay, I have two identical Pentium 4 computers, with identical Pioneer DVD Burners.
The two computers both connect to the router, so in that sense, none of them is master or slave.
It was just a matter of speech.
One of them, in the living room, is what I use most of the time. All stuff burnt on this, will play on both computers.
The bedroom one, which I use less frequently, is the one that burnt a few DVDs which won't play on the living room computer.
But! The story is even weirder because some DVDs will play on the living room computer if the other is turned on. If I turn it off, then it won't read it.
Those DVDs that were Authored (by TMPGEnc DVD Author 1.5) on the bedroom computer, will not read, even if it's turned on.
I know it sounds weird, but this is how it is.