I dont mean for the OS's but lets say you already have both installed on an internal HD and external HD.

Then you go out a buy a 1TB HD, would it be possible to partition 500 GB of it in NTFS format to work in windows, then parition the other half to work in Mac?

I guess so. That is all we have been doing with this dual , quad, or more boot ..isnt it?

Yep but for Snow make sure you use a OSInstall.mpkg that has been modded for MBR. For Distros nothing is needed because they have already been modded. Also if you were meaning just for DATA then you can format the whole thing FAT and this will allow you to have read/write access for both. Just not good for files over 4GB because they corrupt.