Is this a brand new hard drive? If it is new then it probably is formatted to NTFS for Windows. You need to use Disk Utility to partition it to GUID partition scheme and format to Mac extended.
Pondini: Formatting, Partitioning, Verifying, and Repairing Disks... - http://pondini.org/OSX/Home.html
If this is already a Mac drive then you need to tell us for what you use the drive. There are various ways to make it both read and write for you but some work better in some situations than others.