How to Fix: The semaphore timeout period has expired, Error 121


If you encounter the Windows Error: 121, “The semaphore timeout period has expired”, this could have several potential origins:

It could be a hardware connection issue, to or from the drive, such as its physical connection.

Beyond that it could be a network issue, as described in Microsoft’s KB 325487

The network issue may be related to the NIC configuration or speed or duplex settings.

Furthermore, another possibility is that the file system became corrupt, either at source or target, and you could run chkdsk on both drives (source and backup drive) to check for file system inconsistencies.

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