pst-file / ost-file in use
When trying to create a backup copy of my pst-file, I get the following error;
"0x80070021: The process cannot access this file because another process
has locked a portion of this file."
Outlook is closed when trying to make a copy of the file so why is it still in use?
Outlook might be closed but something is hijacking the outlook.exe process which has the lock on the file. As long as that process runs, you cannot do anything with the pst-files.
General causes
Usually this is the result of a certain add-in that you have installed or an application that relies on the outlook.exe process such as fax software or having a mobile device connected to your PC. Outlook is then still running even when all Outlook windows are closed.
Verify if Outlook is indeed still running in the background
Since Outlook 2007 with Service Pack 2 installed, you’ll see the following icon in your Notification Area for additional visual feedback about Outlook’s current state; 
You can also press CTRL+SHIFT+ESC to bring up the Task Manager in Windows and select the Processes tab to see if outlook.exe indeed is still running.
Releasing the lock on outlook.exe
When you close the application that relies on the outlook.exe process, Outlook will completely shut down as well. For instance, when you have a mobile device connected to your computer which syncs with Outlook, it will keep the outlook.exe process running for as long as the mobile device is connected or the sync application is running.
If you just closed Outlook, it could take a while before it actually fully closes. Outlook could be running internal processes on the pst-file (such as Auto-Compact) or waits for another utility to finish its process such as an indexer utility (Windows Desktop Search, Google Desktop Search, Copernic, etc…). If Outlook still hasn’t completely terminated itself after over 30 minutes and the pst-file or ost-file is still locked, it is safe to assume that it won’t close on its own anymore no matter how long you wait.
Terminating outlook.exe and rebooting
You could choose to terminate the outlook.exe process to force the release of the file lock. If you don’t see outlook.exe running and the file is still locked, then try restarting your PC. Since this forces the release of the file lock, you might want to check the pst-file for errors afterwards.
If you are experiencing this issue every time you close Outlook, further troubleshooting is required. For that see the guide Outlook doesn't close.
