When I use PKZip 2.04g with zipmax and run it against my whole rom archive collection (4,814 zip archives), 27 of them fail in Zipmax with "ERROR" and no other explanation.

I can't see any reason why these 27 archives should be any different. I can run any of the other external compressors with no problem at all. I know that PKZip probably doesn't give a better compression rate than 7zip but at this point, I'm just curious why this is happening.

If you want to test this out yourself, here's links to pkzip 2.04g and one of the archives that fail in Zipmax...

http://www.3feetunder.com/krick/mame/hunchbkg.zip
http://www.3feetunder.com/krick/mame/PKZIP.EXE

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Krick