Can't install new Seagate HD on mac -- greyed partion option under disk utilites
In all likelihood, the drive is formatted as NTFS - Window’s proprietary filesystem - which operates as read only in OS X. In order to partition the drive, or even use it, you need to reformat it first. 


Open Disk Utility. Select the hard drive from the sidebar. Click “Erase”
Choose a name for the volume. You must also choose a format and scheme. The default OS X configuration is HFS+ Journaled with a GUID partition map. Unless you require universal compatibility, just go with that format.
Click erase 
Now you’ll be able to partition/modify the drive to your liking.