Writing to External Hard Drive

Hi all. My friend has just bought a 320gb maxtor hard drive and she transferred some data to this using a Windows eek: ) PC. When I connect it to my Macbook I only get read permissions on this, and cannot write any data to it. I have plugged it in to a different Windows machine and it works fine. Any ideas on this? I have checked the permissions when it's connected to a Windows machine and they are set to allow full control for everyone. I also have an external hard drive of my own which works fine on both a Windows XP machine and also my Macbook.

Thanks in advance.