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?

  • I don't think there's any way to do this without a risk of losing all of the data; if you can't back it up, you probably should leave it as is. Commented Mar 2, 2013 at 21:35
  • I pull off the data I need, on Windows if required, then reformat using GUID. Commented Feb 21, 2019 at 21:03
  • Why the answer it's not online? I can see a little bit of "invalid argument mounting disk. Could not mount disk1s2 (...)" wich is pretty similar to mine --- Could not mount disk2s2 with name (null) after erase Error: -69832: File system formatter failed--- this could be useful to me! why deleted? Commented Feb 21, 2019 at 22:06

2 Answers 2


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:

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

    diskutil partitionDisk /dev/disk3 1 GPT HFS+ newdisk R
  3. copy-n-paste the above text to the command line and run

  4. returned to Disk Utility and was able to partition away
  • Wow! Thank you! Worked flawlessly! Thank you again!
    – Robb A
    Commented Sep 9, 2019 at 6:28

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