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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?

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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. – Gordon Davisson Mar 2 '13 at 21:35
up vote 0 down vote accepted

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.

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