I was able to load the mac made rns file to my PC!!!
Mac made rns files contain a header (Mac filetype information), and the file cannot be loaded in the Windows version of Reason.
To understand the difference open any mac and PC made rns file using any text editor (but DO NOT save it) . You will see that the mac file contains a header (which is usually 128 characters/bytes long) then the string "Propellerheads Reason Song File" and then binary data. The PC file starts right away with the "Propellerheads...." string.
The rns file is a binary file and by simply using a text editor (notepad, or vi in unix) to delete the header won't work. You have to strip the header (which is usually 128 characters/bytes long) using binary read/write functions.
I wrote a tool in C that strips the header and creates a new file that can be loaded in the Windows version of Reason.
You can download it from:
http://www.clsp.jhu.edu/people/stavros/reason/drcut.c
No more "Bad Format"

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