I just discovered that I am not able to write to my external HDD (Seagate Free Agent Pro) via my MBP. Are there any solutions to make it work without reformatting the entire external HDD? I have backup files on there already, and I don't want to lose those. I think I read that it has something to do with the formatting of the drive. Can I split the external HDD so that one half works for the mac and one half works for the PC, or will I have to buy another unit specifically for the mac?