How to Format an External Disk on Mac: Complete Guide

Introduction / Why This Is Needed

Formatting an external drive on macOS is a standard procedure that solves many problems: preparing a new drive for use, erasing all data before selling, fixing read/write errors, or changing the file system for compatibility with other operating systems (e.g., Windows). This guide will walk you through the process safely and efficiently using macOS's built-in tools.

Requirements / Preparation

1. macOS version 10.13 (High Sierra) or newer (instructions are relevant for Sonoma, Ventura, Monterey).
2. A connected external drive (USB flash drive, HDD, SSD, SD card) via a Mac port (USB-A, USB-C/Thunderbolt). For reliability, use an original cable or hub.
3. A backup. All content on the target drive will be completely erased. Ensure important files are saved elsewhere.
4. Administrator privileges. Formatting system volumes or some external drives may require admin rights. Log in with an administrator account.

Step-by-Step Instructions

Step 1: Connect the Drive and Open Disk Utility

Insert the external drive into an available port on your Mac. Wait for the system to detect it (an icon may appear on the desktop).

Launch Disk Utility:

• Via Finder: Open the Applications → Utilities folder and launch Disk Utility.app.
• Via Spotlight: Press Cmd + Space, type Disk Utility, and press Enter.

Step 2: Select the Target Drive in the Sidebar

In the left sidebar of the Disk Utility window, you'll see a list of all physical disks and their partitions (volumes).

It is crucial to select the top level—the physical disk itself (usually it has the manufacturer's name, like SanDisk Ultra, WD My Passport, or simply UNTITLED). Selecting a partition (volume) will format only that partition, potentially leaving unusable portions of the drive.

💡 How to tell a disk from a partition? A disk (Physical Drive) usually has a smaller size equal to the drive's total capacity (e.g., 31.5 GB for a 32-GB flash drive). A partition (Volume) is lower in the hierarchy and has the same size as the displayed available space.

Step 3: Configure Formatting Options

After selecting the disk, management buttons will appear in the center of the window. Click Erase (or Delete).

A dialog window with key settings will open:

Step 4: Start Formatting

After configuring the parameters, click the Erase (or Delete) button. macOS will request final confirmation.

What happens: Disk Utility will create a new partition table (scheme) and format the selected partition into the specified file system. A progress indicator will be displayed.

Time: From a few seconds (small flash drive) to 10-15 minutes (large 2-4 TB HDD). Do not interrupt the process or eject the disk.

Step 5: Verify the Result

Upon completion, the disk will automatically mount (appear on the desktop and in the Finder sidebar). Its name will match what you entered in the "Name" field.

Quick verification:

1. Open the new disk in Finder.
2. Confirm it is empty.
3. Press Cmd + I (Get Info) and verify that the "Format" matches your selection (APFS, exFAT).

Result Verification

If the disk appears and is writable—formatting was successful. You can copy a test file (e.g., a small image) onto it and then delete it to check integrity.

For a deeper check (especially for new or suspect drives), you can use the built-in verification:

1. In Disk Utility, select the formatted partition (volume) of the disk.
2. Click the First Aid button.
3. Run the check. A successful completion will confirm the file system's integrity.

Potential Issues

Issue: The disk does not appear in Disk Utility or in the "External" list

• Solution: Check the physical connection. Try a different port (USB-A/USB-C), a different cable, or hub. Restart your Mac. If the disk is not recognized on another computer, the drive itself or its power supply (for HDD) may be faulty.

Issue: The "Erase" button is inactive (grayed out)

• Cause: You selected a partition (volume) instead of the disk, or the disk is a system disk or encrypted.
• Solution: Ensure the top-level item—the physical drive's name—is selected in the sidebar. If it is the macOS system disk, it cannot be formatted due to protection.

Issue: Error "Operation failed" or "Could not complete operation"

• Possible causes and solutions:
  1. The disk is damaged. Try running First Aid on the disk itself (select the top-level disk, not a partition). If that doesn't help, the disk is likely faulty.
  2. Scheme conflict. Try changing the Scheme (e.g., from GUID to MBR and back) and repeat formatting.
  3. Force format via Terminal. This is a last resort. Open Terminal and find the disk's identifier with the diskutil list command. Then execute:

     # WARNING! This command will DESTROY ALL data on the disk!
     # Replace diskX with your disk's identifier (e.g., disk2)
     sudo diskutil eraseDisk FORMAT_NAME FORMAT_TYPE /dev/diskX
     # Example for exFAT:
     # sudo diskutil eraseDisk EXFAT EXFAT /dev/disk2
     

     You will need to enter your administrator password. This forcibly erases the entire partition table and creates a new one.

Issue: After formatting to exFAT, the disk works on Mac but is not recognized in Windows

• Solution: Ensure Windows has the latest updates. In very old versions of Windows (XP without SP3, 7 without updates), exFAT support may be missing. In that case, use FAT32 (with the 4 GB file limit) or install the exFAT driver from Microsoft.