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I was trying to create a new partition on my macbook by booting through Recovery USB using the disk Utility. But I am unable to create a partition since it throws a "Cannot unmount disk" error. I tried to manually unmount the drive but it doesn't work.

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You can't unmount it because the Recovery Partition HD is on that drive. – user3463 Sep 20 '11 at 5:40
up vote 0 down vote accepted

Try Erasing the the existing partition first. If the drive has been claimed by CoreStorage, you won't be allowed to change the partitions until it's clean.

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