How to remove the use of master boot record from an external hard drive? I have a 1TB external hard drive and I want to slice it into different partitions. When I try to do so, I get this error:


  can not modify external drive because it uses master boot record


Is there a way to fix this without deleting the data that I have in it?

I have 700GB of data, and therefore I can not back it up; even when I try to compress it or make a dmg file in Disk Utility. 

I have also tried to use the Disk Management on Windows 7 and still it does not allow me to shrink this.

Is there a way to shrink this partition without losing the data?
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