Backing up files

I recently bought my passport external hard drive for my mac ibook G4 (p/n WDBAAA5000ARD-00 ) my computer has no more disk space my ext hd shows me all my files but wont let me back up files when i click on back up info warning box pops up click on info empty box pops up what am i doing wrong?

This drive is factory formatted in NTFS, this is a Windows file system, you need to reformat your drive, in order to used.!!&p_li=&p_topview=1

ok thanks for your help got it sorted now cheers…