Partitioning external hard drive

You can do what you want, but you don't know how. So, see these articles. Read before going further.

Disk Utility for Mac- Partition a physical disk using Disk Utility

Using macOS Sierra Disk Utility to Partition, Erase Drives

Disk Utility (El Capitan)- Partition a physical disk

Familiarize yourself with all of this. Now, let's go back to the beginning:

Drive Partition and Format

Restart the computer. At the chime press and hold down the Command and R keys to boot from the Recovery HD.

Open Disk Utility in the Utilities' folder.

After Disk Utility loads select the external drive (an out-dented entry with the mfgr.'s ID and size) from the side list.

Click on the Erase tab in the Disk Utility toolbar. A panel should drop down.

In the drop down panel set the partition scheme to GUID. Set the Format type to Mac OS Extended (Journaled.)

Click on the Apply button and click on the Done button when it is activated.

Click on the Partition tab in the toolbar. You will see something like this:

Click on the Add [+] button below the circle. This will create a second partition half the size of the disk. Not quite like the below, but you get the idea.

You now select the partition you want, then move its sizing handle left or right as required to change the size of that partition sort of like what you see above. Be sure to put in a name before clicking on the Apply button so you will know which partition is which.

Select the small partition for your XBox. Set the Format to MS-DOS (FAT), put a Name in, and you will see something like this:

Click on the Apply button. You now have an XBox partition formatted for DOS and one formatted for OS X.