Hi Lane_h15,
I understand that you're trying to partition an external hard drive via Disk Utility, but are unable to do so via the Partition button. This typically indicates a situation in which you'll need to have Disk Utility rewrite the drive's partition map. This can be done by selecting the root drive on the left in Disk Utility (Likely indicated as 'Samsung...'), clicking Erase, and choosing your desired partition map scheme and a format for the drive's primary partition. Once erased, you should now be able to click Partition and add and remove partitions normally.
You can erase all volumes on a device and create new ones. For example, if you want to change the partition map of a USB, Thunderbolt, or other storage device from Master Boot Record to GUID Partition Map. If you simply want to erase a volume, see Erase a volume.
WARNING: Erasing and partitioning a device destroys all data on all volumes of the device.
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Select the device in the sidebar you want to erase and repartition, then click the Erase button 
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Note: You can’t erase the device that contains your startup volume.
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Enter a name for the initial volume.
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Click the Map pop-up menu, then choose a format for the initial partition.
For MS-DOS and ExFAT partitions, the name must be eleven characters or less.
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GUID Partition Map: Used for all Intel-based Mac computers.
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Master Boot Record: Used for Windows partitions that will be formatted as MS-DOS (FAT) or ExFAT.
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Apple Partition Map: Used for compatibility with older PowerPC-based Mac computers.
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Click Erase, then click Done.
Disk Utility (El Capitan): Partition a physical disk
https://support.apple.com/kb/PH22240
Regards!
