How to remove the use of master boot record from an external hard drive?

Use Gparted to resize your existing partitions. Make a bootable disc or USB stick. And be careful, Gparted can easily destroy your data. If this doesn't work then you will probably struggle to find any other option.

There is no way to my knowledge to change MBR to GPT without destroying all of the partitions and their associated data.

Here are the steps I took (note: my drive was blank so I don’t the impact on any data)

Get drive name:

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

Replace Windows MBP with GPT:

5. Open Terminal

6. Replace the “disk3” in the command below with #4 above:

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

7. copy-n-paste the above text to the command line and run

8. returned to Disk Utility and was able to partition away