Note: This has been successfully tested on a Mac OS X 10.6.8 (Snow Leopard).
Mac OS X doesn't detect NTFS formatted external disks by default. However, it is possible to install some (open source) drivers to read the content of an external disk, and it's even possible to write on a NTFS partition!
First you need to install MacFUSE: http://code.google.com/p/macfuse/downloads/list
Second you need to install Tuxera NTFS for Mac: http://www.tuxera.com/products/tuxera-ntfs-for-mac/
After a reboot, the attached external disk (NTFS formatted) will be detected by your Mac and it is even possible to write data on it.