The file you are trying to use has be locked by another program and cannot be accessed. For example, if Microsoft Word has a file open, it will prevent another program from accessing the file.
To proceed with actions on the file or folder, the application using it must release it. Either save or close the open file.
If you continue to get a warning, try closing the open application.
If you had an application crash since powering on, Windows may think a file is still in use, when it no longer is. A reboot should clear the issue.
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