Disk Utility on Mac: Everything It Can Do (and What It Shouldn't)

Disk Utility is one of those Apple apps that’s been around since the OS X Public Beta and somehow still feels half-finished. The interface is clean, the buttons are clearly labeled, and yet half of what you’d want to do with a disk requires Terminal because Disk Utility quietly refuses or hides the option behind menus you’d never find by clicking.

That said, when it works, it works well. First Aid catches real problems. Erase actually erases. Partition can carve up a drive without breaking what’s on it (sometimes). Knowing what Disk Utility can and can’t do — and when to reach for diskutil or asr instead — is the difference between fixing a problem in 30 seconds and corrupting the wrong volume.

The Layout

Disk Utility lives at /System/Applications/Utilities/Disk Utility.app. When you open it, the left sidebar shows storage devices, and there’s an important distinction in how they’re displayed.

By default, the View menu is set to “Show Only Volumes.” This hides the underlying devices and containers. You’ll see “Macintosh HD” and “Macintosh HD - Data” and any external drives — but not the APFS containers or physical disks they live in.

Switch to View → Show All Devices (or Cmd+2). Now the full hierarchy appears:

• The physical disk (e.g., “APPLE SSD AP1024Z Media”)
• The container inside it (e.g., “Container disk3”)
• The volumes inside that container

The “Show All Devices” view is the right default for any serious work. You can’t repair or partition properly without seeing the full structure.

First Aid: What It Does

The First Aid button runs fsck_apfs (for APFS volumes) or the equivalent for HFS+ or other formats. It checks the filesystem’s metadata for inconsistencies and repairs them when possible.

What it catches:

• Cross-linked files (two files claiming the same block)
• Orphaned inodes (files with no parent directory)
• Incorrect free-space accounting
• Corrupted catalog records
• B-tree damage (the index structure APFS uses for directories)

What it doesn’t catch:

• Actual data corruption (bad bytes within otherwise-correct files)
• SSD-level issues (failing flash cells, wear-out)
• Logical errors (a file you accidentally overwrote yesterday)

First Aid is read-only on the boot volume in normal mode — it won’t write changes to the running system volume. To actually repair the boot volume, you need to be in Recovery mode (hold Cmd+R at boot on Intel, or hold the power button on Apple Silicon to reach Startup Options).

When First Aid Fails

If First Aid reports problems it can’t fix, you have a few options:

1. Boot into Recovery and run First Aid from there. The boot volume isn’t mounted live, so repair has more freedom.
2. Use fsck_apfs -y /dev/diskNsM from Terminal in Recovery — sometimes the GUI’s verbosity hides specific options.
3. Backup, erase, restore. When First Aid can’t repair, the filesystem damage is bad enough that erasing and restoring from a backup is faster and more reliable than continuing to fight.

If First Aid fails repeatedly across multiple repair attempts, the issue may be hardware. Run Apple Diagnostics by holding D at boot (Intel) or holding the power button and choosing Diagnostics from Startup Options (Apple Silicon).

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Erase: The Format Options

The Erase button presents a few choices. The format dropdown matters a lot:

• APFS — modern Apple file system, default for SSDs on Sierra+
• APFS (Encrypted) — APFS with FileVault-style encryption
• APFS (Case-sensitive) — same but distinguishes file casing
• APFS (Case-sensitive, Encrypted)
• Mac OS Extended (Journaled) — HFS+, the older format, still useful for backwards compatibility
• Mac OS Extended (Journaled, Encrypted)
• Mac OS Extended (Case-sensitive, Journaled)
• MS-DOS (FAT) — for compatibility with old Windows or simple devices, max file 4 GB
• ExFAT — for cross-platform USB drives, no 4 GB file limit

When to pick what:

• Internal drive → APFS (and consider Encrypted if it’s not already FileVault-protected)
• External drive used only with Macs → APFS for performance and snapshots
• External drive shared with Windows → ExFAT, accepting the lack of journaling
• Time Machine destination → APFS (Sonoma+) or APFS (Encrypted)
• Bootable installer → Mac OS Extended (Journaled)

The Scheme dropdown for whole-disk erase is usually GUID Partition Map. Apple Partition Map is for ancient PowerPC Macs. Master Boot Record is for cross-platform with old systems.

Erase vs. Erase Volume Group

On modern macOS, you might see “Erase Volume Group” as well. This is for the boot volume specifically, where the system and data volumes are linked into a “volume group.” Erasing the volume group nukes both at once — what you’d want when truly resetting a Mac.

Erasing just one volume of a group leaves the other in a weird state. If you’re trying to reset, use the volume group option (or use the much more reliable “Erase All Content and Settings” in System Settings, which Apple introduced in Monterey).

Partition

The Partition button lets you carve up a disk into multiple sections. On APFS, this is more nuanced than it used to be — APFS has containers with multiple volumes that share space, while partitioning splits the physical disk into separate containers.

When to partition vs. add volumes:

• Partition when you need true isolation — e.g., a partition for Boot Camp / Windows, or for running a different macOS version with its own free space
• Add Volume (a button that appears for APFS containers) when you want a separate volume that shares space with the rest of your drive

Adding an APFS volume is non-destructive and instant. Partitioning is potentially destructive and slower, since it has to relocate data.

Mount, Unmount, and Eject

The toolbar’s Mount/Unmount button appears when you select a volume that’s eligible. The difference between Unmount and Eject:

• Unmount — removes the volume from the file system but leaves the device connected
• Eject — unmounts and prepares the device for safe removal

For external drives, always Eject before unplugging. For internal drives, you’ll typically only Unmount (you can’t eject the drive your Mac is running from).

Restore: Cloning a Volume

The Restore button is one of Disk Utility’s most useful (and least-known) features. It lets you clone one volume onto another using asr (Apple Software Restore) under the hood. Block-level copy, very fast.

Steps:

1. Select the destination volume (the empty one you’re cloning to)
2. Click Restore in the toolbar
3. In the dialog, select the source volume

The destination must be at least as large as the data on the source. Restore will erase the destination during the copy.

This is genuinely useful for:

• Cloning an external drive to a larger one
• Making a one-time copy of a partition for archival
• Setting up an identical environment on another disk

Caveats:

• Doesn’t work for the boot volume in normal mode (use a clone tool like Carbon Copy Cloner for that)
• The clone is a snapshot in time, not an ongoing sync
• May not preserve all extended attributes in some configurations

Image Operations

The File menu has commands you’ll never see in the toolbar:

• New Image from Folder — bundle a folder into a .dmg
• New Image from — full-disk image of a volume
• Convert — change a .dmg between read-only, read-write, compressed, encrypted formats

For making encrypted archives of sensitive data, “New Image from Folder” with AES-256 encryption is a solid built-in option. Set the format to “read/write” if you want to add files later, or “read-only” / “compressed” for archival.

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What Disk Utility Won’t Do

Where Disk Utility falls short and you reach for diskutil (the command-line tool) instead:

Repairing on Intel Macs in non-Recovery boot

Sometimes you need to run fsck_apfs against a specific volume before mounting. Disk Utility’s UI insists on mounted volumes for First Aid. From Terminal:

diskutil unmount /Volumes/External
sudo fsck_apfs /dev/disk4s2

Forcing a stuck volume to release

If Disk Utility says “Couldn’t unmount,” try:

sudo diskutil unmountDisk force /dev/disk4

Inspecting AFPS snapshots

Disk Utility doesn’t show snapshots. The CLI does:

diskutil apfs listSnapshots /

Resizing volumes

Resize via the GUI works for partitions but not always reliably for APFS volumes within a container. From Terminal:

diskutil apfs resizeContainer disk3 1.5T

(That resizes the container, not a volume — APFS volumes within a container don’t have fixed sizes.)

Listing all attached storage with structure

diskutil list

Gives you a complete tree of every disk, partition, and APFS container/volume. Sometimes Disk Utility’s sidebar misses something.

Risky Operations to Avoid

Some things Disk Utility will let you do that you shouldn’t:

• Erasing the wrong volume. Once it’s started, the data is gone. Verify the device path (disk2 vs. disk3) before clicking. Look at the size and filesystem to confirm you’ve selected the right thing.
• Repartitioning the boot drive on Apple Silicon. Don’t. The boot drive’s partition layout on Apple Silicon includes signed boot artifacts that Disk Utility may not handle gracefully if you re-carve.
• Removing the Recovery partition. It looks like dead weight (it’s only 600 MB) but it’s how you’d recover from a corrupted system.
• Decrypting a FileVault volume from Disk Utility on a system that’s running. Use System Settings → Privacy & Security → FileVault for that, not Disk Utility.

When the Disk Itself Is Failing

Disk Utility is a software tool. It assumes the underlying storage is functioning. If your SSD is dying:

• First Aid may report errors that come back after each repair
• File operations get slow or hang
• The system reports SMART status as “Failing” (older Macs) or just refuses to mount the drive

For SMART status:

diskutil info disk0 | grep "SMART Status"

If it says anything other than “Verified” on a drive you trust, replace it.

For sustained read errors or write failures, the drive is past saving. Get your data off, and the drive itself is for the recycler.

What This Buys You

Disk Utility is fine for:

• Erasing a drive
• Running First Aid on a removable volume
• Creating a .dmg or encrypted image
• Quick-and-dirty disk inspection
• Adding a volume to an APFS container

Reach for diskutil (CLI) when:

• You need granular control over operations
• The GUI refuses to do what you need
• You’re scripting a recovery
• You’re working with snapshots or unusual configurations

For boot volume repair, neither is enough on its own — you’ll usually be in Recovery mode using one or both. But the everyday Disk Utility workflow on macOS is genuinely productive once you flip to “Show All Devices” and stop trusting “Show Only Volumes” to give you the full picture.