This is due to the currrent Mac OS's lack of write ability for NTFS formatted drives. The DataLocker comes pre-formatted with widely used Windows NTFS filesystem. To use exclusively in a Mac environment simply format the drive on your Mac using HFS+ filesystem.
If you plan to use the DataLocker between a Windows and Mac environment you have two options:
1) Install Fuse for OSX
For more info on Fuse for OSX visit link: http://osxfuse.github.com/
*You must check the box "MacFUSE Compatibility Layer" at the installation screen."
Install NTFS 3G Driver
For more info on NTFS 3G Driver visit link: http://sourceforge.net/projects/catacombae/
*At the caching selection option, pick no caching best results.*
2) Reformat the drive in the exFAT format. Please read this wiki for disadvangtages of using exFAT format. Some older operating systems may not be able to use the new exFAT filesystem. To use exFAT in Windows XP environment, download and install the KB955704 update.
The DataLocker supports multiple partitions and formats allowing you to create numerous partitions with different formats depending on your needs.