i would suggest using ntfs-3g software if you like to use your external HDD on both windows and apple. however the only drawback from ntfs-3g is that you can't use it to back up your files using time machine.

To use NTFS-3g u just need to dl and install these 2 files onto your apple and it should work.
1) MacFUSE - http://code.google.com/p/macfuse/
2) NTFS-3G - http://macntfs-3g.blogspot.com/

hope this helps.