Outlook cannot open .pst file which was in use by an unknown program

We have a small "workgroup" network - 3 users, 3 computers running XP and MS Office 2003, 1 Buffalo TeraStation NAS, no servers

On each PC, there is a U: drive mapped to the user's personal folder on NAS

One of the users intermittently got an error when opening his Outlook
"Unable to open your default e-mail folders. The file U:\Mail\Outlook.pst is in use and could not be accessed. Close any application that is using this file, and then try again"

The problem happened on Sep 20 for the first time. I restarted NAS, restarted the computer, no joy. I killed the tasks of MS Security Essentials and Google Desktop, etc, that seemed to fix the problem. So I told the user to leave Outlook open, do not close it.
I checked this computer yesterday, Outlook worked fine. I ran scanpst.exe against the pst file on NAS, I opened and closed Outlook a few times, the problem did not show up at all.

Today, the user called me because the problem came back again.
I spent an hour but could not fix it regardless what i did. I even disabled all third party services and programs from MSCONFIG, but still no joy.

I installed "unlocker" but it said that nothing was opening the pst file, while there must be something!

The other 2 computers are totally fine.

I am wondering if you know any program like "unlocker" that is able to detect which program is using the .pst file.
Or if you have seen this problem and know the solution?



Noob Whisperer
How big is the .pst file?
Have you considered moving the .pst file back to the user's local machine and pointing Outlook at that location?
As I recall earlier versions of Outlook didn't do very well with large .pst files (over a gig) especially .pst files stored on the network, rather than locally.
Lastly, when you close Outlook, double check task manager and make sure the Outlook is not still running.

Restore through the Import and Export Wizard, it is unusual restoration, but quite effective

-Open the Import and Export Wizard

Outlook 2007 and previous

File-> Import and Export…

Outlook 2010

File-> Open-> Import

Outlook 2013

File-> Open & Export-> Import/Export

-Choose Import from another program or file.

-Choose Personal Folder File (.pst).

-Browse to the location where you’ve restored the pst file. For instance: D:\My Documents\My Outlook\Temp

-Set the options to “Include subfolders” and “Import items into the same folder in:” and select the folder that is listed as your Outlook Today folder set in the dropdown list.

-Press Finish to complete the import.

Restart Outlook and remove the export.pst file from the Temp directory if desired.

Outlook stores user information in form of Outlook PST file and seems your Outlook PST has been corrupt. Microsoft has given a inbuilt utility (Scanpst.exe) to repair corrupt Outlook PST but utility fails in case of scanning severe PST corrupt. I was also facing corrupt PST file issue and resolved it easily by using this software being an Outlook user I would suggest you to opt for Stellar Phoenix Outlook PST Repair to make your PST file free from corruption. Tool works well.

This happen when you have moved the PST file from its orignal location and replaced it again.
  • Click in Start > Control Panel > Mail
  • The Mail Setup dialog shall open up, click on Data Files button.
  • Now you may be confronted to 2 situations:
    • Path to .pst file is already present----> select the mail folder (highlight) and click on "Set as Default".
    • .pst file is absent -----> click on Add -----> browse for mail folder and click on "Set as Default".
  • Close the Mail Setup dialog.

Note that: Prior to these manipulation, make sure that your .pst file is not locked in read only mode.
  • Right-click on the file > Properties > Uncheck "Read-Only" attribute.


New Member
If Outlook PST file gets damaged due to any reasons, use a default program, known as Inbox repair tool (scanpst.exe) to repair Outlook PST file. With the help of this tool, you can get effective solution.

Emanito hecona on about.me

Download one such ideal PST Repair software to easily repair damage or corrupted Outlook.ost file.

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