Err.. yeah.. so, there you are, you need to copy something from your apple mac to your external disk, but it wont allow you.. the disk is not full and it works perfectly on windows.??

I recently encountered this problem, the drive was formatted as NTFS (and why wouldn't it be?) turns out macs can read files off NTFS drives, but they cannot write to an NTFS drive.

Ok, so what now? You can format your external hard drive as FAT32 partition or NTFS-3G

NTFS-3G is an open source, freely available NTFS driver for Linux, Mac OS X, FreeBSD, NetBSD, Solaris, BeOS and Haiku. It is licensed under the GNU General Public License. Once installed, it will allow you to write to your NTFS drive without a problem!