Have you use the external drive on a PC?
You can only read NTFs drives not write to them with out a driver such as NTFS-3G
How-to: Read and Write NTFS Windows Partition on Mac OS X - Stepcase Lifehack click here
If it's only going to be used on a Mac, then Mac OS Extended (Journaled) will work perfect.
Basically open up Disk Utility (Finder Toolbar > Go > Utility's > Disk Utility), Click the drive in question, click 'Erase' > Volume Format > Pick either Mac OS Extended (Journaled) or Mac OS Extended > Click Erase.
Now your done. However this will destroy any data on the drive, and will only be able to be used on Mac.