Find out what to do when your Mac does not recognize an external USB drive, hard disk, or flash drive — and how to make it appear in Finder and Disk Utility.

Step 1: Launch Terminal

Hold Command (⌘) + Space → enter Terminal and press Return

Step 2: Execute the Drive Repair Script

Copy the command below into the Terminal window and hit Return

How this script works

Refreshes the macOS USB subsystem and disk arbitration service to detect connected drives

Repairs common disk permission and mount issues that prevent drives from appearing

Does not format, erase, or modify any data on your external drives

Why your USB drive is not showing up

There are several reasons why an external drive may not appear on your Mac. The macOS disk arbitration service — which is responsible for detecting and mounting storage devices — can occasionally become unresponsive or fail to recognize a newly connected drive. File system permission errors, corrupted directory structures, or incompatible formatting (such as NTFS without a driver) can also prevent a drive from mounting. Additionally, faulty USB cables, unpowered hubs, or loose connections may cause intermittent detection failures that appear to be software issues.

What gets fixed

Disk arbitration — restarts the macOS service responsible for detecting and mounting external storage devices

USB subsystem — refreshes the IOKit USB driver stack to re-enumerate connected devices

Mount permissions — repairs file system permissions that may prevent a drive from being mounted in Finder

Drive directory — verifies and repairs the disk's directory structure to resolve corruption issues

Finder integration — forces Finder to refresh its sidebar and desktop so that detected drives appear immediately

Is it safe?

Yes. The repair script only interacts with macOS system services and does not write any data to your external drives. Your files on the USB drive, external hard disk, or flash drive remain completely untouched. The script restarts detection services and repairs mount points — operations that macOS performs internally during a normal system restart.

When should I try other steps?

If the drive still does not appear after running the script, try using a different USB cable or port, connecting the drive directly to your Mac instead of through a hub, or checking Disk Utility (Applications → Utilities → Disk Utility) to see if the drive is detected but not mounted. For drives formatted as NTFS, you may need a third-party driver to enable read/write access on macOS.

Tips to prevent USB drive issues

Always eject drives properly using the eject button in Finder before physically disconnecting them

Use high-quality USB cables — cheap or damaged cables are the most common cause of intermittent detection failures

Avoid connecting drives through unpowered USB hubs — external hard drives often require more power than a hub can provide

Format drives as APFS or exFAT for the best cross-platform compatibility between macOS and other operating systems

Keep macOS updated — Apple regularly includes USB driver improvements and bug fixes in system updates

Published Date: March 9, 2026