okay.. here is the step to make it able as Time Machine hard drive..
1. Open Disk Utility
2. Select the new hard drive
3. Go to the Partition tab
4. Choose the number of volumes (normally one) from the Volume Scheme dropdown
5. Click on the Options… button
6. Choose GUID Partition Table or Apple Partition Map depending on whether your Mac is Intel-based or PowerPC-based.
7. Click Apply.