it sould like your mac ntfs driver are not working.
mac osx can only read ntfs normally
drivers are required to write to ntfs as well as format a hdd to ntfs
1 Get NTFS tools here for OSX :- http://macntfs-3g.blogspot.com/ 2 Install and reboot , you will see an extra control panel in the System preferences for NTFS Preferences on the Mac. "See the control panel in the :- Apple menu / System preferences , NTFS control panel pane " .3 . Launch Disk utilities on the Mac , use Spotlight search to find the application and launch with a left click on the name found in the search results. 4 Once NTFS tools are installed on your Mac you can format NTFS in the Disk utilities application on the Mac , Found in Applications / Utilities .
you maybe using other ntfs drivers already ? make sure they are installed or running ok ,
http://www.paragon-software.com/home/ntfs-mac/ also see paragon ntfs drivers for mac osx , there are very good .
Does this help.
You can check your disk is working correctly by using the hdd with any windows pc running xp or above . copy and file to and from the hdd using windows .
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please use firefox on this site .