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Format it in Windows with FAT32. This is the only way to share files on a portable hard drive between Windows and Mac OS. If you did it NTFS, Mac OS would only be able to read from it, and not write to it.

The downfall is that FAT32 only supports files up to 4GB.
 
I have a firewire/USB 2.0 drive that I formatted to the Unix option in OSX, and it is readable/writeable on my PC (XP), my other PC (Linux/Ubuntu), and my mac...
 
also

theres a program out there that can read mac formats on pc, i cant remember what its called though, sorry
 
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