Mac OS Sierra - Disk Utility will not let me partition an external hard drive
I've just encounter the same problem, where Partition icon is grayed out. I found the solution.
Simply launch this command in terminal and the partition icon will not be grayed out anymore :
diskutil erasedisk hfs+ External GPT /dev/disk2

For clarity, in the above command, the path argument starts from /dev, not &quot;External GPT &quot;
However, this command will erase the disk, make a backup if needed before !
Source : Apple Discussion - Alberto Ravasio