macdrive is a utility that will allow you to mount a macosformatted drive on a pc. Worth every penny.
There are also mac utillities that will allow you to write to NTFS drives. Just do some googling. There are paid and free solutions.
Have you tried formatting your External HD in ExFAT? This will allow you to transfer very large files for use with PC. Keep in mind that this only works with Lion and Mountain Lion, not earlier versions of Mac OSX. Try it out on a 'sacrificial' XHD to see if it works and get back to us.