Need to create a new partition, or re-format an external drive? There’s no need to hunt down paid partition managers or disk-management boot disks: your Mac includes a built-in partition manager and disk management tool known as Disk Utility.
Disk Utility is even accessible from Recovery Mode, so you can partition your Mac’s hard drive without having to create and load up any special bootable tools.
Accessing Disk Utility
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To access the Disk Utility in macOS, just press Command+Space to open Spotlight search, type “Disk Utility” into the search box, and then press Enter. You can also click the Launchpad icon on your dock, click the Other folder, and then click Disk Utility. Or, open a Finder window, click Applications in the sidebar, double-click the Utilities folder, and then double-click Disk Utility.
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To access the Disk Utility on a modern Mac—regardless of whether it even has an operating system installed—reboot or boot up the Mac and hold Command+R as it boots. It’ll boot into Recovery Mode, and you can click Disk Utility to open it up.
In Recovery Mode, macOS runs a special sort of recovery environment. This allows you to use Disk Utility to wipe your entire drive—or repartition it.
Partition Drives and Format Partitions
Disk Utility shows internal drives and connected external drives (like USB drives), as well as special image files (DMG files) that you can mount and access as drives.
On the left side of the window you’ll see all mounted volumes.
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This annoyingly leaves out empty hard drives, but click Views > Show All Devices in the menu bar and you’ll see a tree of drives and their internal partitions. Each “parent” drive is a separate physical drive, while each little drive icon below it is a partition on that drive.
To manage your partitions, click a parent drive and select the “Partition” heading. You can adjust the partitioning layout scheme here. You can also resize, delete, create, rename, and reformat partitions.
Note: Many of these operations are destructive, so be sure you have backups first.
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If you want to repartition your system drive, you’ll need to do this from within Recovery Mode, with one exception: APFS volumes. APFS is Apple’s new file system, the default on solid state drives as of macOS High Sierra, and it’s got all sorts of clever tricks up its sleeve. One of them: volumes on the same drive pool storage space, meaning you’ll see two separate drives in Finder, but won’t have to manage how much storage space each volume uses. To add a new APFS volume, simply select your system drive, and then click Edit > Add APFS in the menu bar. You’ll see the above prompt.
First Aid Repairs File System Problems
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If a hard drive is acting up, Disk Utility’s First Aid function is the first thing you should try. This feature checks the file system for errors and attempts to correct them, all without much intervention from you.
Simply click the drive you want to check, then click the “First Aid” button. Be warned that these checks can take a while, and running them on your system drive will leave you with an unresponsive computer until it’s done.
Secure-Erase a Partition or Drive
The Erase button allows you to erase an entire hard disk or partition. You can also choose to only erase its free space.
You can use this feature to securely wipe a hard drive. Click a drive, then click the “Erase” button, then click “Security Options” to select a number of passes to overwrite the drive with. One pass should be good enough, but you can always do a few more if you feel like it. The maximum number is unnecessary.
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Note that this feature will only be useful on mechanical drives, as you shouldn’t be able to recover deleted data from a solid state drive. Don’t perform a secure erase on a solid-state drive, such as the ones built into modern Mac Books—that will just wear down the drive for no advantage. Performing the “fastest” erase of the internal drive from recovery mode will erase everything.
Create and Work With Disk Images
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Click the File menu in Disk Utility and use the New menu to create blank disk images or disk images containing the contents of a folder — these are .DMG files. You can then mount that disk image file and write files into it. This is particularly useful because you can encrypt that DMG file, creating an encrypted container file that can store other files. You can then upload this encrypted DMG file to cloud storage locations or save it on unencrypted removable drives.
The Convert and Resize Image buttons will allow you to manage that disk image from the Disk Utility window.
Copy Volumes and Restore Disk Images
The Restore feature allows you to copy one volume to another. You can use it to copy the contents of one partition to another, or to copy a disk image to a partition.
You can also create a disk image that contains an exact copy of an entire partition. Select the drive you’d like to create an image of, and then click File > New Image > Image From [Partition Name].
You can later restore this disk image file to a partition, erasing that partition and copying the data from the disk image to it.
RAID Setup
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The Disk Utility also allows you to set up RAID on a Mac: just click File > RAID Assistant in the menu bar. Combine disks and partitions into one or more RAID sets and choose whether you want to mirror, stripe, or concatenate your data. This is an advanced feature most people won’t need to use, but it’s there if you need it.
Mirroring (RAID 1) means data you write to the RIAD is stored on each partition or drive for failsafe purposes. If one drive dies, your data is still available elsewhere.
Striping (RAID 0) will alternate disk writes between one drive and the other for faster speed. However, if one of the drives fails, you’ll lose all the data — so it’s getting more speed at the expense of less reliability.
Concatenation (JBOD) allows you to combine different drives as though they were one, useful in certain circumstances.
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The Disk Utility included with Mac OS X is powerful, and it should handle all the functions you need it to perform. It’s a bit like the Disk Management tool built into Windows, but more capable and, thanks to Recovery Mode, easier to access from outside the operating system.
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