How to remove the use of master boot record from an external hard drive?
Here are the steps I took (note: my drive was blank so I don’t the impact on any data)

Get drive name:


Run Disk Utility
Click External Drive
Click Info icon
Scroll to bottom and find the Parent disks name ex. disk3


Replace Windows MBP with GPT:


Open Terminal
Replace the “disk3” in the command below with #4 above:

diskutil partitionDisk /dev/disk3 1 GPT HFS+ newdisk R

copy-n-paste the above text to the command line and run
returned to Disk Utility and was able to partition away