Using Disk Utility on macOS: A Complete Guide

Introduction / Why You Need This

Disk Utility is a built-in macOS tool for managing all types of storage devices: internal SSDs/HDDs, external flash drives, memory cards, and even disk images. With it, you can:

• Format and rename disks.
• Create, resize, and delete partitions.
• Check and repair file systems (First Aid).
• Create and restore backups as disk images (.dmg).
• Manage RAID arrays (for advanced users).

This guide will help you confidently use Disk Utility for everyday tasks: preparing new media, fixing errors, and creating backups.

Requirements / Preparation

Before you begin, ensure:

1. You have administrator rights on your Mac (a password will be required for some operations).
2. All important data on the disk you plan to format or modify has been backed up elsewhere. Partitioning operations will result in complete data loss on the target disk.
3. For external disks, they must be properly connected via USB, Thunderbolt, or USB-C.
4. It is recommended to have a stable power source (laptop plugged in), especially when working with the system disk.

Step-by-Step Guide

Step 1: Launching Disk Utility and Selecting a Disk

Open Disk Utility using one of these methods:

• Through Finder: Applications → Utilities → Disk Utility.
• Through Spotlight: press Cmd + Space, type Disk Utility, and press Enter.

In the left sidebar, you will see a list:

• Internal disks (usually Apple SSD... or Macintosh HD).
• External disks and their partitions.
• Disk images (.dmg files mounted as virtual disks).

Important: For operations on the entire disk (formatting, changing partition scheme), select the top item — the physical disk (e.g., Apple SSD AP0256J), not the partition below it (e.g., Macintosh HD). For checking a partition (First Aid), you can select the partition.

Step 2: Viewing Disk Information

After selecting a physical disk, detailed information appears on the right:

• Capacity: total size.
• Type: Physical Disk, Logical Volume, Free Space.
• Used/Available: how much space is occupied and free.
• Partition Scheme: GUID Partition Map (recommended for Mac) or Master Boot Record (for Windows compatibility).
• File System Format: APFS (default for modern Macs), Mac OS Extended (Journaled) (for older HDDs), exFAT (for cross-platform media), MS-DOS (FAT) (legacy).

Review this data to understand your disk's current layout.

Step 3: Formatting and Creating Partitions

Formatting (complete erase) a disk:

1. Select the physical disk (not a partition) in the left column.
2. Click the Erase button in the top toolbar.
3. Fill in the fields:
   • Name: new disk name.
   • Format: choose the appropriate one. For a system or primary Mac disk, use APFS. For a flash drive used on both Windows and Mac, use exFAT.
   • Scheme: GUID Partition Map (unless Windows BIOS compatibility is needed).
4. Click Erase. Confirm the action. The process takes from a few seconds to several minutes depending on disk size.

Creating multiple partitions on one disk:

1. Select the physical disk.
2. Click the + (plus) button in the Partition Scheme section.
3. In the graphical editor that appears, you can:
   • Resize partitions by dragging the boundary between them.
   • Add new partitions (click +).
   • Delete partitions (click -).
4. For each partition in the list, specify Name, Format, and Size.
5. Click Apply. Disk Utility will re-partition the disk and format each partition according to the specified parameters.

⚠️ Important: Creating/modifying partitions will completely destroy all data on the disk. Ensure you have a backup.

Step 4: Using First Aid for Checking and Repairing

First Aid checks file system integrity and attempts to fix any errors found.

1. Select the disk or partition you want to check.
2. Click the First Aid button in the top toolbar.
3. Click Run.
4. The process begins. For non-system disks, it runs immediately. For the system disk (Macintosh HD), Disk Utility may offer to create a temporary boot volume and restart your Mac to perform the check in the background. Agree to this.
5. After completion, you will see a report: Operation successful or Disk Utility found errors that it could not fix. In the latter case, more serious intervention may be needed (restore from Time Machine, reinstall macOS).

Step 5: Creating and Restoring Disk Images

Disk images (.dmg) are single files containing an exact copy of a disk or folder's structure and data. Ideal for backups and software distribution.

To create a disk image:

1. Select a physical disk, partition, or even a folder.
2. From the File menu, choose:
   • New Image → From [name of selected item] (for full disk/partition).
   • New Image → From Folder... (for selective folder backup).
3. Specify the save location, file name. The image format is usually UDIF (default). For compression, select Compressed.
4. Click Save. The process may take time proportional to the data volume.

To restore data from an image:

1. From the File menu, select Restore from Image....
2. Specify the .dmg file you want to restore.
3. Select the target disk (warning! all data on it will be overwritten!).
4. Click Restore. Enter the administrator password if the image is encrypted.

Verifying the Result

After each operation, check the outcome:

• After formatting/creating partitions: The disk should appear on the Desktop and in Finder with the name you specified. Open it to confirm it is empty.
• After First Aid: In Disk Utility, the status should change to OK. The report should show no critical errors.
• After creating an image: The .dmg file should exist at the specified path. Double-click it — it should mount as a virtual disk, showing its contents.
• After restoring from an image: The target disk should contain an exact copy of the data from the image.

Possible Issues

• Error: 'Operation failed' during formatting.
  • Cause: The disk is in use by another process, physically faulty, or write-protected (e.g., lock switch on a card reader).
  • Solution: Close all applications that might access the disk. Try restarting your Mac. For memory cards, check the lock switch. If the error persists, the disk may be faulty.
• Disk Utility does not see an external disk.
  • Cause: Bad cable/adapter, faulty port, disk not receiving power.
  • Solution: Try a different cable, different port, or another computer. Check if the disk's indicator light is on. Open System Information (About This Mac → System Report) and look for the disk under USB or Thunderbolt. If it appears there but not in Disk Utility, the issue may be with drivers or Disk Utility itself.
• First Aid cannot fix an error.
  • Cause: Severe file system structure damage or physical media issues.
  • Solution: If it's the system disk, boot into Recovery Mode (start your Mac and immediately hold Cmd + R), launch Disk Utility from there, and try First Aid again. If it's data, you may need specialized recovery software or to replace the media.
• Cannot resize a partition (gray slider).
  • Cause: The partition contains unmovable data (e.g., swap file or system file) or uses a different file system (e.g., NTFS).
  • Solution: Delete unnecessary partitions and create new ones. For APFS system partitions, resizing is usually supported. For Windows partitions (NTFS), use Windows utilities.