Apple Tips:  Using Disk Utility

3.  Add  a  Partition

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Partitions must occupy contiguous physical space on a disk, so you may be able to add a partition to a disk, if there’s enough space left on it.

Actually, what you do is split an existing partition, to make room for a new one.

You can do this with the Disk Utility app, in your Applications/Utilities folder.  If you’re on Leopard or later, you can partition your internal HD while you’re running from it. 

But if you’re on an earlier version of OSX, to partition the disk you’re running from, you must boot from a different source, such as an external HD with OSX installed, or your OSX Install disc.  See the pink box in item #1 for instructions.

Either way, be sure you’re using the major version of OSX you’re running -- Leopard, Snow Leopard, etc.

Partitions to be used by Boot Camp must be added and managed by Boot Camp Assistant only.  See Mac 101: Using Windows on your Mac via Boot Camp for details.

Note:  Effective with Lion (10.7.x), if a disk contains an encrypted partition, no partitions on that disk can be added, resized, or deleted by Disk Utility.  If you're familiar with UNIX and Terminal, you may be able to do this with the diskutil command.   OSX Versions will show what version you're on.

  1. a.When Disk Utility starts, select the drive in the sidebar (the line with the size and make/ID).

  2. If your disk doesn't appear, see If a disk doesn't appear in the sidebar.

  1. b.Click the Partition tab. Disk Utility will show a chart of the current partition(s),  with the amount used in each shaded in light blue.
  2. c.Click the partition you want to split.  It will be outlined in blue.

  3. d.Click the plus sign at the bottom. Disk Utility will show it split into two parts, and name the new one the same as the old, but with "_2" appended.

  1. e.Click the new partition; it will be outlined in blue.
  2. f.Give it a name (to show on your desktop/Finder sidebar).

  1. g.Adjust its size, either by dragging the divider between the new and old partitions, or typing the desired size into the Size box.

  2. h.Set the Format as desired, via the pop-up menu.      See Disk Formats for details.

  3. i.If you want to add more partitions, repeat the process:  steps c through h.

  4. j.When you have it the way you want it, click Apply.

  5. k.Read the confirmation prompt
    carefully, then click Partition.  
  6. l.The drive will be re-partitioned, unless DU can't get enough contiguous space;  then it will send you an error message, and not change anything.


You can buy some 3rd-party apps that will move data around so you can do the partition even when Disk Utility can't, such as iPartition.  But they all strongly recommend that you back-up the data first, "just in case" something goes wrong.  Well, if you're going to do that, you might as well simply do the copy, then erase and reformat the disk, then copy the data back. 

Here's how to partition without erasing, via Disk Utility:

Partitions must occupy contiguous physical space on a disk.

If the existing partition has ever had more data than it does now, then some things were deleted, the remaining data may be scattered over it, so there may not be enough contiguous space left for the new partition.

When you Get Info on a partition, or select the partition in Disk Utility’s sidebar, you’ll see the amounts Used and Available.  This is, of course, the total amount of data, and the total empty space.   If that data is all at the beginning of the partition, you can reduce the partition to that size; if it’s scattered, you can’t.

But when you select the top line of the drive in Disk Utility’s sidebar and click the Partition tab, then select a partition as shown below, the amount shown as Available may be smaller -- it’s the amount of contiguous empty space at the “end” of the partition -- the maximum available for a new partition. 

It seems odd to see different values for the amount Available on the same volume in different places, but that’s the reason.

Before the actual partitioning starts, Disk Utility will do a Verify Disk on the partition being split.   If you're adding a partition to your startup disk, your Mac may respond very slowly while that's in progress.  If a problem is found, you'll get a message to the effect that the verify failed, and the disk won't be partitioned.

If it fails, run Repair Disk (not permissions) on it, per item #6, then try again.