Complete Guide to Disk Utility on macOS: Management and Recovery

Introduction / Why This Is Needed

Disk Utility is a built-in system application in macOS for managing storage devices. With it, you can:

• Check and repair file systems (First Aid).
• View drive health (S.M.A.R.T. status).
• Create, modify, and delete partitions (volumes).
• Format drives in various file systems (APFS, HFS+, exFAT, FAT32).
• Create and mount disk images (.dmg files).
• Securely erase drives (multi-pass overwriting).

This is the primary tool for basic maintenance and preparation of storage devices in macOS. This guide covers the main scenarios for using this utility.

Requirements / Preparation

1. macOS version 12 Monterey or newer. The interface and some features may differ in older versions (e.g., before macOS Catalina).
2. Administrator privileges. Operations on system or external drives (especially formatting) may require an administrator password.
3. Backup. Before any operation that changes disk structure (creating/deleting partitions, formatting), always create a backup of important data to another storage device or via Time Machine.
4. Physical connection. For external drives, ensure they are properly connected via USB/Thunderbolt and appear in Finder.

Step-by-Step Guide

Step 1: Launching and Interface of Disk Utility

Launch the application via Finder → Applications → Utilities → Disk Utility or through Spotlight (Cmd+Space).

In the left column, you will see the device tree:

• Physical disks (e.g., Apple SSD AP...). Selecting this item allows you to work with the entire storage device (S.M.A.R.T., partitions, full formatting).
• Partitions (Volumes) (e.g., Macintosh HD, Data). Selecting a partition allows you to work with that specific volume: First Aid, formatting, creating an image.
• Disk images (.dmg files) that are already mounted in the system.

The top toolbar contains the main buttons: First Aid, Partition, Erase, Create Image, Restore.

Step 2: Checking and Repairing Disks (First Aid)

The First Aid function is equivalent to chkdsk in Windows or fsck in Linux. It checks and attempts to fix file system errors.

1. In the left list, select the physical disk (not a partition) for a comprehensive check or a specific partition to check only its file system.
2. Click the First Aid button.
3. In the dialog window, click Run.
4. The process will begin. For the system disk (Macintosh HD), the system may request a reboot and will perform the check in pre-boot mode (before macOS loads). This is normal.
5. After completion, you will receive a report: The disk appears to be OK or Errors were found and repaired. If errors cannot be fixed, or the check fails, this is a serious indicator of potential disk problems.

⚠️ Important: First Aid cannot repair physical damage (bad sectors) or disk controller issues. It only works with the logical file system structure (APFS, HFS+).

Step 3: Viewing Disk Health (S.M.A.R.T.)

S.M.A.R.T. (Self-Monitoring, Analysis and Reporting Technology) is a self-diagnostic technology for storage devices.

1. Select the physical disk (top level in the list, e.g., APPLE SSD AP...).
2. Look at the status on the right, under the disk name. It will be displayed on the information panel itself.
   • Not Supported — for some external disks or virtual devices.
   • Failing or Verified / Passed — good status.
   • Failing or Failed — critical signal. The drive may fail soon. Immediately back up all data and replace the disk.

Step 4: Creating, Modifying, and Deleting Partitions

Creating a new partition in free space:

1. Select the physical disk (e.g., Apple SSD AP...).
2. Click the Partition button (or the + in the partition scheme).
3. A blue rectangle representing the current partition will appear on the disk map. Click + in the center to add a new partition.
4. Specify:
   • Name: Arbitrary (e.g., Data).
   • Format: APFS (recommended for SSD) or Mac OS Extended (Journaled) for compatibility.
   • Size: Drag the slider to set the desired size.
5. Click Apply. The system will recalculate partitions and create the new volume. The process may take several minutes.

Modifying or deleting an existing partition:

1. Select the physical disk and open the Partition tab.
2. On the map, click the partition you want to modify/delete.
3. Use the - button (delete) or change partition sizes by dragging their borders.
4. Deleting a partition: Click -, then Apply. Data on this partition will be permanently deleted.

💡 Tip: To resize the system partition (Macintosh HD) on the same disk where the system resides, it's better to use Disk Utility in Recovery mode (boot with Cmd+R at startup). This reduces the risk of damaging the active system.

Step 5: Formatting a Disk

Formatting creates a new file system on the selected partition or disk, destroying all existing data.

1. Select a partition (volume) or physical disk (if you want to erase all partitions).
2. Click the Erase button (in the top toolbar).
3. Fill in the parameters:
   • Name: New disk/partition name.
   • Format:
     • APFS — default for macOS (especially SSD).
     • Mac OS Extended (Journaled) — for compatibility with older macOS (< High Sierra) or HDD.
     • exFAT — for exchanging large files between macOS and Windows.
     • FAT32 — for compatibility with all systems, but with a 4 GB file size limit.
   • Scheme:
     • GUID Partition Map — standard for all modern Macs with Intel and Apple Silicon processors.
     • Master Boot Record — for compatibility with very old systems (Windows XP and earlier).
4. Click Erase. Confirm the action. After completion, the disk/partition will appear in Finder with the new name.

Step 6: Creating a Disk Backup Image (DMG)

A disk image (.dmg) is a compressed or uncompressed file-copy of a disk or partition's contents. Useful for archiving, transferring, or creating a bootable copy.

1. Select the partition or physical disk you want to archive.
2. In the application menu, select File → New Image → Image from [selected name].
3. In the dialog window:
   • Save As: Specify the path and filename (e.g., Backup_2024_02_14.dmg).
   • Image Format:
     • UDIF (compressed) — standard, takes up less space.
     • UDIF (uncompressed) — no compression, faster creation/mounting.
   • Encryption: (Optional) Choose 128-bit AES or 256-bit AES to password-protect the image.
4. Click Save. The creation process may take a long time depending on the data volume. The finished image will mount as a virtual disk for verification. After checking, unmount it via Eject in Finder.

Step 7: Securely Erasing a Disk (for Selling/Disposal)

Simple formatting does not physically delete data. To securely destroy information on an HDD (not an SSD!), use the Erase function with the Security Options setting.

1. Select the physical disk or partition.
2. Click Erase.
3. Click the Security Options... button.
4. Choose the method:
   • Just Erase Files — fast, but data is recoverable.
   • Erase Free Space — overwrites only unused blocks.
   • Erase Entire Disk (for HDD) — 1, 3, 7, or 35 passes. 35 passes — maximum security, but very slow (a day or more).
5. Click OK, then Erase.

⚠️ Critically important for SSD: On modern SSDs (all Macs from 2012 onwards), do not use multi-pass overwriting! This wears out the memory without real benefit. For an SSD, a single format with the Erase Data option enabled (available in the Erase window) is sufficient. The hardware Secure Erase command for SSDs is executed automatically by the controller when formatting in APFS.

Verifying the Result

• For First Aid: The report window should state The disk appears to be OK. In System Report (Applications → Utilities → System Report), under S.M.A.R.T. Status for each disk, the status should be Verified or Passed.
• For creating a partition: The new partition should appear in Finder and in Disk Utility's left list as a separate volume.
• For formatting: The disk should appear in Finder with the name and format you specified. Test it by trying to write and read a file.
• For creating an image: The .dmg file should be created in the specified folder. Double-click it to mount and verify data accessibility.

Troubleshooting

Problem: First Aid cannot repair the disk, reports Error: (-69874) or similar. Solution: This often indicates serious logical corruption or a physical problem. 1) Back up data via Time Machine or manual copying if the disk is still readable. 2) Try running First Aid from Recovery mode (boot with Cmd+R). 3) Be prepared to replace the disk.

Problem: Disk Utility does not see an external disk or shows it with an error. Solution: 1) Check the cable and connection. 2) Try a different port/cable. 3) Go to System Report → USB/Thunderbolt and check if the device is detected at the hardware level. 4) The disk may be damaged or require power (for HDDs, use a powered USB hub).

Problem: Cannot format/erase the system disk (Macintosh HD). Solution: The system volume from which macOS is booted cannot be modified in a running system. You need to:

1. Restart your Mac.
2. At startup, press and hold Cmd+R to enter Recovery mode.
3. Launch Disk Utility from the Utilities menu in the top menu bar.
4. Select the system disk and perform the needed operations.

Problem: When trying to create a partition or resize, an error Not enough space or Cannot resize appears. Solution: This means there is no contiguous free space of the required size on the disk. 1) Using Disk Utility in Recovery mode, try to merge all partitions into one. 2) Alternatively, delete unnecessary partitions/data to free up a single contiguous block. Disk Utility cannot "shift" existing partitions to free up space—it must be contiguous.