I've yet to see this trick. Here's why. Each system has its own ideas about file systems.
Bob
I'm looking to buy an external HD (Lacie 80Gb Mobile) and I was wondering if I can use it to back-up my files from both laptops - iBook mac OS X (my wife's)and Dell Windows XP (mine).
If this is possible, then are there anything special that I would need to do?

Chowhound
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GameSpot
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TechRepublic