unable to partion an exteranl drive with a GUID table

Here's what you should do: Factory reset of your Mac - Apple Support. Follow the instructions as provided. When you are ready to install macOS, do the following. This only works for Late-2011 or later models.

Internet/Network Recovery of El Capitan or Later on a Clean Disk

     If possible back up your files before proceeding.

1. Restart the computer. Immediately after the chime hold down the (Command-Option-Shift-R) keys until a globe appears.
2. The Utility Menu will appear in from 5-20 minutes. Be patient.
3. Select Disk Utility and click on the Continue button.
4. When Disk Utility loads select the target drive (out-dented entry with type and size info) from the side list.
5. Click on the Erase button in Disk Utility's toolbar. A panel will drop down.
6. Set the partition scheme to GUID.
7. Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
8. Click on the Apply button, then click on the Done button when it activates.
9. Quit Disk Utility and return to the Utility Menu.
10. Select Install OS X and click on the Continue button.

This should install the original version of macOS that came with the computer. Once the installation has completed the computer should restart. After it restarts do nothing but shut down the computer. It is now ready for the next owner.