Checking Disk Health on macOS: A Complete Guide

Introduction / Why This Matters

Disk health (SSD or HDD) is a critically important factor for macOS stability. A failing disk causes sudden freezes, read/write errors, data loss, and slow system performance. This guide will help you diagnose your storage drive's condition using macOS's built-in utilities without installing third-party software. You will learn to distinguish between logical and physical errors and respond appropriately to warnings.

Requirements / Preparation

Before you begin, ensure:

1. You have administrator privileges (required for some Terminal commands).
2. The disk is not in use by other processes (close all applications working with files on the target disk).
3. To use smartctl (advanced S.M.A.R.T. diagnostics), install Homebrew and run:

   brew install smartmontools
   

4. Important: If the disk is already behaving erratically (frequent crashes, boot errors), do not run lengthy checks—create a backup immediately.

Step-by-Step Instructions

Step 1: Identify Your Disk's Identifier

Find the exact system identifier for your disk, which will be used in commands.

1. Open Terminal (via Spotlight Cmd+Space → type Terminal).
2. Enter the command:

   diskutil list
   

3. In the output, find your main system disk. Typically, this is:
   • For Apple Silicon Mac: APFS Container (identifier), inside which is disk1s1 (macOS volume).
   • For Intel Mac: disk0 (physical disk) or disk0s1 (volume).
4. Write down the identifier. Example for Apple Silicon: disk1s1. For external disks, look for external, physical in the description.

Step 2: Check via Graphical Interface (Disk Utility)

The simplest method for most users.

1. Open Disk Utility (/Applications/Utilities/Disk Utility.app).
2. In the left sidebar, select the disk itself (not the volume!). For the system disk, this will be your Mac's name (e.g., "Macintosh HD"). If you only see volumes, click "View" → "Show All Devices."
3. Click the "First Aid" button in the top toolbar.
4. In the dialog, click "Run".
5. Wait for completion. The process includes:
   • Checking the file system structure (APFS/HFS+).
   • Checking S.M.A.R.T. status (if supported).
6. Result:
   • "Done" — disk is OK.
   • "The file system was repaired" — logical errors found and fixed.
   • "Failed" — serious problems detected, possibly physical.

Step 3: Check via Terminal (Advanced Options)

Use Terminal for detailed diagnostics and an S.M.A.R.T. report.

3.1. Quick File System Check

Run this, replacing diskX with your identifier from Step 1:

diskutil verifyDisk /dev/diskX

Example:

diskutil verifyDisk /dev/disk1s1

This command checks the integrity of the APFS container and all volumes. It is safe and does not modify data.

3.2. S.M.A.R.T. Status Check (SSD/HDD only)

This requires smartmontools (see Preparation).

sudo smartctl -a /dev/diskX

Key lines in the output:

• SMART overall-health self-assessment test result: PASSED — disk is healthy.
• FAILED or Pre-fail — disk is near failure. Replace immediately!
• SMART Attributes: — attribute table. Pay attention to:
  • Reallocated_Sector_Ct (reallocated sectors) — value >0 indicates wear.
  • Current_Pending_Sector (pending sectors) — >0 is critical.
  • Uncorrectable_Sector_Ct (uncorrectable sectors) — >0 is fatal.

Note: On some newer Macs (especially Apple Silicon), direct S.M.A.R.T. access may be restricted. In this case, smartctl may return Unavailable - device lacks SMART capability. Trust Disk Utility's built-in check instead.

Step 4: Interpreting Results and Actions

Verifying the Fix

After repair:

1. Restart your Mac.
2. Launch applications that previously malfunctioned. Ensure errors like "could not write file" or "disk not recognized" are gone.
3. To check I/O speed (if you suspect a slow disk), use blacktree or AJA System Test from the App Store. Compare results with benchmarks for your model.

Potential Issues

Issue 1: diskutil returns "Unable to verify this disk" (disk is busy)

Solution: Close all applications, including Finder (if any windows are open from that disk). Boot into Recovery Mode (Cmd+R at startup) and run Disk Utility from there to check the system disk.

Issue 2: smartctl does not see the disk or says "Unavailable"

Solution: This is normal for some modern SSDs (especially Apple's). Fully trust Disk Utility results. For external disks, ensure they are connected directly, not through a hub.

Issue 3: Disk Utility hangs on "First Aid"

Solution: Checking a large disk with many errors can take a long time. Allow 2-3 hours. If it's a definite hang (progress unchanged for 30+ minutes), reboot into Recovery Mode and try again.

Issue 4: Errors return after repair

Solution: This is a clear sign of physical wear. Do not waste time on repeated checks. Back up immediately and replace the drive. Continuing to use it will lead to data loss.

Issue 5: No "First Aid" button for the disk

Solution: You selected a volume, not the physical disk. In Disk Utility, click "View" → "Show All Devices." Then select the top-level item (the physical disk), and the button will become active.