Mac OS Sierra - Disk Utility will not let me partition an external hard drive Within disk utility, the partition icon is grayed out.  If I mouse over it, it reads:  "Disks with Master Boot Record Partition Maps cannot be partitioned".  The history on this drive:  It is a 2 TB Seagate external hard drive that the Mac store reformatted for me to use with Time Machine. I had a problem right after I bought my Mac and we did this to backup my machine and literally restore it to a new machine.  Since then, I have used it with time machine and now want to re-partition it so I can store backups of additional files to the other partition (once it is created).  The current single partition is "Mac OS Extended (Journaled)".  Any ideas how to address this issue?  I would prefer not to have to erase the disk as part of the process (if possible).
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