I have successfully used the MacFuse/NTFS-3G stuff to be able to write to an NTFS external USB drive. (I also access my bootcamp partition that way.) It works, but the authors have plenty of warnings about possible corruption. If you don't need the drive to be shared between Mac and PC, then I would reformat to a Mac OS format. If you need to share it, the MacFuse/NTFS-3G thing will work, but you might be advised to have the drive scan checked on a Windows box occasionally. If you have PC systems around, you can just remotely access the drive (shared from Windows with Read/Write permissions) and use it that way. It will be much slower, but because Windows is actually handling the drive access, corruption should not be an issue.