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.