How to fix Error 121 on Windows 11

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Windows 11 Error 121: “The Semaphore Timeout Period Has Expired”

Why This Error Happens

Error 121 occurs when a data transfer between two devices (commonly over network shares, SMB connections, or USB/external drives) takes longer to acknowledge than the system’s allotted timeout window, causing Windows to abandon the connection. This is typically caused by unstable network connections, faulty or outdated network adapter/USB drivers, or corrupted SMB (Server Message Block) protocol settings. Physical issues like damaged cables, failing hardware, or interference on wireless connections can also trigger this timeout.

Step-by-Step Resolution Guide

  1. Update Network/USB Drivers
  • Press Win + X → select Device Manager.
  • Expand Network adapters (or Disk drives for USB issues), right-click your device → Update driver → Search automatically for drivers.
  • Restart your PC after updating.
  1. Disable SMB Auto-Tuning (for network share issues)
  • Open Command Prompt as Administrator.
  • Run: netsh interface tcp set global autotuninglevel=disabled
  • Restart the computer and re-test the file transfer.
  1. Check and Repair Network Adapter Settings
  • Go to Settings → Network & Internet → Advanced network settings → Network adapter options.
  • Right-click your active adapter → Properties → Configure → Advanced tab.
  • Locate Speed & Duplex and set it manually to match your router/switch (e.g., “100 Mbps Full Duplex”) instead of “Auto Negotiation.”
  1. Inspect Physical Connections and Hardware
  • Swap Ethernet cables or USB cables/ports to rule out physical damage.
  • If using Wi-Fi, move closer to the router or switch to a wired connection temporarily to isolate the issue.
  1. Run System File Checker (SFC) and CHKDSK
  • Open Command Prompt as Administrator.
  • Run: sfc /scannow
  • Then run: chkdsk C: /f /r and restart if prompted.

Alternative or Verification Steps

  • Verify the fix: Attempt the same file transfer or network operation that previously failed; monitor Event Viewer (eventvwr.msc) under Windows Logs → System to confirm Error 121 no longer appears.
  • Alternative solution: If the issue persists on a mapped network drive, try disabling Large Send Offload (LSO) in the network adapter’s Advanced properties, as this feature is a known contributor to semaphore timeout errors on certain NIC models.

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