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I have an external HD with a mix of Mac, NTFS and Linux partitions. I wanted to use it to backup some partitions from my MacBook HD, and partially succeeded, copying a 100MB partition into an empty 300MB partition. Unfortunately Disk Utility won't let me resize partitions, or create any new partitions, giving the following message.

You can't resize partitions on this disk because it uses the Master Boot Record partition scheme.

Is there any way around this? None of my Windows partition programs can do anything with the Mac OS Extended partition. Can I convert to GUID Partition Table and still use with Windows XP?

I don't want to lose the content of the NTFS partition, which contains years worth of backups.

PS Is this a new issue with Mountain Lion? I thought I had done these on Snow Leopard.

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The only way I know is to conver the MBR disk to a GPT disk. I would backup your data incase something happened. What third-party tools have you tried? –  Ramhound Oct 8 '13 at 12:42

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Converting from MBR to GPT will render the disk unusable from Windows XP. A partial exception: The GPT Mounter tool claims to enable Windows XP to mount GPT partitions; however, I've never tried it, so I can't comment on how well it works. If you do want to try an MBR-to-GPT conversion, my GPT fdisk (gdisk, sgdisk, and cgdisk) can do the job. So can some Windows-only tools, but I don't recall the details about them.

Overall, your best bet is probably to use GParted under Linux; you mention the disk has Linux partitions, so presumably you've got a Linux installation somewhere. If not, you could use a Linux emergency disc. In any event, GParted can resize most filesystems on both MBR and GPT disks.

One important caveat: Resizing partitions is inherently dangerous, so if you've got any irreplaceable data on the disk, back it up before you do anything. It sounds like you're using the disk as a backup storage medium, so you might instead simply be prepared to immediately make new backups if something goes wrong.

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Thanks for the informative answer (and even more informative documentation on your site). My 2007 XP laptop won't support GPT, and I don't want to lose the backups of my genealogy data, so maybe it is time to get a new disk. –  Milliways Oct 9 '13 at 2:32

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