the 3.1gb is Layer0. the other 2gb is Layer1. only 1 file is on Layer1, its called GT4L1.VOL. windows explorer doesn't show it. you can see it if you make an iso image of the whole game (use dvd decrypter) then look at both layers using a program such as apache 2.
a direct copy of GT4 won't work because the second layer is burned in PTP, dvd writers can only burn OTP on both layers. this basically means the data is burned backwards, so when the game looks for GT4L1.VOL all the data is in the wrong place.
there are 2 ways of fixing these media checks:
1) a DMS4 modchip with toxic 0.3 bios which fixes the checks on-the-fly
2) making an iso of GT4, extracting the iso, patching the files, then playing using the patched files
however number 2) there has a problem - so far its only possible to play these patched files using a hard drive (also requires another file patching to enable the game to be played from just 1 layer, as obviously a hard drive counts as 1 layer). so far you can't play from a dual layer backup as its proving difficult to find a way to put these patched files back onto a disk with the layer break in the correct place
im sure someone will figure out either how to burn a dual layer disk properly, or make a DVD5 rip of the game in the near future. personally i'd prefer a dual layer solution, as its such a big game that surely a rip would either be impossible to play, or missing so much as to be undesirable
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