UPDATE - i've done a bit of learning about .MSI files this afternoon, the .msi file on doom 3 disk 1 was looking for the next disk at certain points in the installation even if the files it needed were present. it looked for the specific volume labels too.
after finding, downloading and learning how to use an msi editor i managed to change it to look for the same volume label for each disk (the label of the dvd im going to make)
this could be useful info for anyone wanting to back up a multiple disk game to a single dvd
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essential vaaapp vaporizer