'Operation not permitted' on firewire/NTFS external HD

[CB]

I have an external firewire drive with an NTFS partition mounted thus:

	/dev/sda1 on /mnt/ext1 type ntfs (rw)

When I try and create/delete any files or directories under /mnt/ext1, I 
get 'Operation not permitted' (all files & dirs currently there were 
placed under windows xp). I can do these operations fine on a vfat 
partition on the same drive.

Any ideas?