View All R&D Articles

“Semaphore Timeout Period Has Expired:” What It Means

February 2, 2026

The “Semaphore timeout period has expired” error (System Error 121) occurs when your computer attempts to transfer data to or from an external drive or network location and fails to receive a response within a set timeframe. You’re most likely to see this message when moving large files over a network or while using external storage devices like USB hard drives and flash drives.

In this article, we will examine why Windows triggers this communication timeout, how to troubleshoot the underlying hardware or network issues, and how to prevent permanent data loss.

What Is a Semaphore Timeout, Anyway? 

A semaphore is a signaling mechanism that manages access to a shared resource by multiple processes. It ensures that only one process uses a specific hardware resource at a given time to prevent data corruption.

When you see a timeout period has expired notification, the operating system is reporting that it sent a request to a device but didn’t get the signal back before the timeout period expired. Usually, that indicates a physical connection failure, a driver conflict, or a failing storage device.

Common Causes for Error 121

A semaphore timeout might occur for a few reasons:

  • Network Instability: If you are transferring files to a Network Attached Storage (NAS) device or a server, any momentary drop in the connection can cause the semaphore to time out.
  • Failing Hardware: A failing read/write head on a hard drive or degraded flash cells in a USB stick can cause the device to hang while trying to access data. 
  • Driver Conflicts: Outdated or corrupted drivers can lead to a breakdown in communication between the OS and the hardware.
  • File Size Limitations: Attempting to move extremely large files (often over 4GB) to a drive formatted with the FAT32 file system can sometimes trigger this error (but usually, you’d get a more descriptive error message).

If you’re encountering a semaphore timeout error for the first time, you can simply try the transfer again — starting with a single, small file if possible. If that doesn’t work, keep reading. 

Resolving a Semaphore Timeout Period Error

1. Verify the Physical Connection

Make sure that you’re transferring files to a storage device that’s reliable and functional. Some simple tips: 

  1. Unplug the external storage device and wait ten seconds.
  2. Connect the device to a different USB port — preferably one located directly on the computer motherboard rather than a front-panel port or a USB hub.
  3. Swap the USB cable for a known working cable.
  4. If using a network drive, restart your router or switch to clear any temporary packet collisions.

2. Update the System Configuration

If the hardware connection is solid, try the following:

  1. Open Device Manager and expand the Universal Serial Bus controllers section.
  2. Right-click each USB Root Hub and select Uninstall device.
  3. Restart the computer to allow Windows to automatically reinstall the most stable drivers.
  4. Temporarily disable any third-party antivirus software.

Note that this is specific to USB-related issues; if you’re transferring over another type of connection, refreshing the USB drivers is unlikely to resolve the problem.

3. Check the Integrity of the Storage Media

Logical errors on the disk can prevent the semaphore from releasing. To check for these, you can use the steps below — but if you need any data from the device, do not run chkdsk. Doing so could cause extensive corruption.

  1. Press the Windows Key and type cmd, then run it as an administrator.
  2. Type chkdsk X: /f (replacing X with your actual drive letter) and press Enter.
  3. Wait for the process to complete.

If you hear any unusual noises (such as grinding or whirring), stop and turn the drive off immediately. Repeatedly forcing the computer to communicate with a degraded hard drive can lead to media damage (scratches on the disk platters).

Professional Data Recovery Solutions

If you are unable to access your files due to a semaphore timeout error, we can help. We provide a risk-free evaluation for all media and operate under a no data, no charge guarantee. Our goal is to ensure that your critical files are recovered safely, quickly, and with the highest degree of security.

Contact our experts at 1-800-237-4200 or submit a case online for a free evaluation.