Permission of external disk under Mac OS

OS X can't write to NTFS natively - you'll need to install a package like MacFuse/NTFS-3G or Tuxera NTFS to write to NTFS partitions.

From the BootCamp Apple Support KB:

During installation, the Microsoft Windows installer asks me to format the Windows partition using NTFS or FAT. Which should I use?

If the partition is 32 GB or smaller, you can use either FAT or NTFS. If it's larger than 32 GB, or you are installing Microsoft Vista, then you can only format it using NTFS. Mac OS X can read and write FAT volumes, but only read NTFS volumes. Refer to the Microsoft Windows documentation if you are not sure which best suits your needs.