I recently ran into an issue which would not allow me to add a partition to a drive attached via USB. I don’t remember the history of this disk, but the buttons/widgets in OS X’s Disk Utility to add, remove, or resize a partition were grayed out. They worked, however, on other internal and external disks. Attempting the same task from the command line resulted in an error message, something along the lines of “diskutility had an error”; not much help there. A quick DuckDuckGo search turned up a StackExchange post regarding the same issue. The resolution is to erase the disk before using it.
diskutil eraseDisk JHFS+ [volumeName] [device]