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My macintosh HDD 500GB has about 350GB data on it. I want to create a 100GB partition and leave mountain lion on that and then move my data to the other 400GB partition. However when I go to create the partition the smallest I seem to be able to make it is 350GB. I'm wondering is this limited because of the data on the disk already?

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If so what approach do I take? Move everything onto an external HDD and then create a partition? Because when I moved everything off (but not empty trash) I still was limited to 350GB partition, its like the data was still there? Can anyone tell me if I move the data to an external drive and empty the trashcan will I get the ability to partition to a smaller size to just house OSX?

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If you want to re-partion your drive, the best way to do that would be to boot up from an external disk. This will allow you to re-partion the internal HD, from my understanding. When using Disk Utilities from the internal drive, it's very limited in what it can do to the internal drive. Like what's been said before, back up your drive fully and then boot up from an external drive that has Disk Utilities on it.

If you don't have a bootable disk with Disk Utilities on it, you can download the Recovery Disk Assistant from Apple's support site here, which has Disk Utilities on it.

You should be able to create a bootable flash drive from that download. Boot up from the flash drive and use Disk Utilities (from the flash drive) to re-size your partition.

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Yes, you're exactly right. You need to move your data to an external drive, delete it (only what you moved!) from your hard drive, then make the partition and migrate your data to it. If you have enough external storage, you should probably back up your whole drive before doing this in case something goes wrong.

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Hi, but when i moved it it didn't allow me to, would this be because i need to empty trash and restart machine? – Pete2k Jan 29 '13 at 11:31
You must empty the trash before the disk's allocation tables reflect that you've cleared out additional space. – Nevin Williams Apr 12 '13 at 15:09

Boot to a Linux Live CD and use GParted. It allows you to resize without destroying contents. I use Ubuntu 12.10 to boot a Intel Mac. Once you have a second partition move some of your data. Then use gparted to reduce the size of the original partition an increase the size of the new one. If you do this with a external drive you are moving all the data twice at usb speeds. This way you are moving it at sata speeds only once. Of course it does take some time to do the resizes.

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There is a possibility of a failure, although I have never had it happen yet and I resize partitions with gparted just about everyday – Kevin Jan 29 '13 at 6:06

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