How-To: Managing partitions with Disk Utility
<p>There may be times where you've either decided to partition a drive or are finished with a partition and wish to remove it and dedicate the space to another partition. The disk management tool Disk Utility, which comes with OS X, contains most of the t
There may be times where you've either decided to partition a drive or are finished with a partition and wish to remove it and dedicate the space to another partition. The disk management tool Disk Utility, which comes with OS X, contains most of the tools necessary for managing partition of drives.
In the past, partitions were relatively static and resizing them was more of a hack than something routinely done. For Windows users, one of the first utilities that supported live partitioning, called "Partition Magic", seemed to primarily work with a little luck and failure meant a lost volume and all the data on it. Therefore the general suggestions were to avoid this for important data and set up a new partition table from scratch if you wanted to resize. Even though live partitioning has advanced and is supported by Disk Utility, we still strongly recommend you at least back up your data before altering the partition scheme of your drives, and consider starting the partition scheme over from scratch if possible to prevent loss of data and disk problems if things go wrong. Regardless of your approach, please back up your data before touching the partition table of your drive.
Creating partitions:
One of the main reasons for partitioning these days is to set up a Bootcamp partition for running Windows or another operating system on your machine. While you can do this manually, the Bootcamp setup assistant is the best way to handle the creation of a partition for installing Windows. However, if you would like to partition your drive manually, such as for creating a scratch disk or a data storage partition, you can do this using Disk Utility.
By default, your Mac comes with one partition that uses the full space available on the drive. If you launch Disk Utility and select the device itself (not the partition named "Macintosh HD", but the device above it in the list), you will see a tab called "Partition" appear. This will allow you to view the partitions on the drive, and you can see that "Macintosh HD" takes up the full space of the "Volume Scheme" section. In this section, you can select this partition and resize it by either typing in a desired size, or by dragging the bottom-right corner of the partition in the "Volume Scheme" section. You cannot make the drive smaller than the data that's currently on it, which is indicated by the purple area.
While this will resize the current partition, you will need to add a new one to make use of the freed-up space. Clicking the " " (plus) button will add a new partition, and dragging the separator between partitions will resize them both. For convenience, Disk Utility supports setting a multiple-partition scheme using the drop-down menu, but you cannot do this on the boot volume, and additionally, doing this will rewrite the whole partition table and destroy all data on the disk.
Partition Table Types
There are a variety of partition table types that are supported by OS X, which you may need depending on the specific use of the drive. These options are available by clicking the "options" key when setting up a new partition table. When you are changing an existing partition table and keeping at least one volume in it, you will not be able to change the partiton table type.
? GUID
The GUID partiton scheme is the default for use with Intel-based macs, and is the most compatible for use with booting an intel mac system. While you can boot from an "APM" partition on an intel mac, we recommend you use GUID if possible. In addition, GUID is the partition type needed if you are planning on using a volume for Time Machine backups.
? APM
The APM or "Apple Partition Map" partition type is the legacy partition scheme used on older PowerPC Macintoshes. If you are planning on creating a boot volume for a PowerPC mac, you will need to use this partition scheme.
? MBR
The last available partition scheme is Master Boot Record, which is a legacy scheme used in Windows PC computers. This will rarely need to be used for Mac-related purposes, but will allow you to create boot partitions that are more compatible with some PCs, which might be useful for drive troubleshooting purposes. Our recommendation is to use this partition scheme for creating a non-bootable drive that you plan on using to transfer files from a relatively older PC.
Deleting and resizing partitions:
Removing partitions is as easy as creating them, but remember that any partition that is removed will have its data lost, so be sure to back up any important files on a partition before deleting it. Once again, if running Bootcamp you can use the setup assistant to remove Windows and restore the used disk space to the Mac OS partition, but this can be done manually with Disk Utility for both Bootcamp and other partitions. Keep in mind that for any two partitions, you will need to remove the one coming after it on the disk in order to resize the partition before it. You cannot remove partitions that exist ahead of another one on disk, and cannot remove partitions that contain the active system software.
Select the partition to delete, and either press the delete key or click the "-" (minus) button. Then drag the current partition to fill the available space in the Volume Scheme, and apply the settings.
Fix disk problems by partitioning:
Sometimes problems with partition tables can prevent a disk from being accessed properly and give unfixable errors to volumes in the partition table. If you are experiencing volume errors that cannot be fixed with Disk Utility, repartitioning the drive is one way to wipe the whole drive clean and rebuilding the data structure on the drive. Depending on the nature of the problem, and whether or not important data is present on the drive, using third-party disk tools to check for and fix partition problems may be a more conservative approach than repartitioning, but if all else fails, repartitioning is the last option before replacing the drive.
With all the relatively cheap external USB and FireWire drives being used these days, partitions can be more easily corrupted as drives are constantly attached and disconnected (especially if they are not properly unmounted before being disconnected). If you are suddenly experiencing problems accessing data on a drive, and have failed at fixing the error with Disk Utility, try running third-party data recovery and disk repair utilities before resorting to repartitioning. Get your data off the drive first if you can, and only use repartitioning to fix the drive itself and get it functional again for future use.
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