If you want to fully share files between Mac & PC you need it to be formatted as FAT32. Although NTFS works great on XP you can't write to it on Mac... only read. And of course any Mac filesystem can't be read on a PC without 3rd party software.
Usually external disks come formatted for FAT32 so you should be good to go already. I know the Seagate I bought was already FAT32.
I had a similar problem with a External HDD I had, it had 2 partitions, 1 FAT32, 1 NTFS, OSX didnt like that fact, so I was forced to delete the partitions in windows and format the drive under OSX.
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