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?

Thanks

Antivirus scanners, lync (communicator), sync software are the common applications that access pst data and cause this error but it could an erroneous error message caused by opening a pst file stored on the network. Opening a pst file that is on a network drive can cause the pst file to become corrupt and is not recommended. 

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 youve 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.

More sources you may visit to know about alternative ways of outlook emails recovery...

https://social.technet.microsoft.com/Forums/en-US/22303144-33ca-4faf-8e79-90039c3879f9/outlook-recovery-data?forum=officesetupdeployprevious

http://www.filerepairforum.com/forum/microsoft/microsoft-aa/outlook/1478-pst-file-error-at-opening-outlook

https://www.repairtoolbox.com/outlookrepair.html Outlook Repair Toolbox

• Marked as answer by Basheer Mahdavi 7 hours 27 minutes ago

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 youve 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.

More sources you may visit to know about alternative ways of outlook emails recovery...

https://social.technet.microsoft.com/Forums/en-US/22303144-33ca-4faf-8e79-90039c3879f9/outlook-recovery-data?forum=officesetupdeployprevious

http://www.filerepairforum.com/forum/microsoft/microsoft-aa/outlook/1478-pst-file-error-at-opening-outlook

https://www.repairtoolbox.com/outlookrepair.html Outlook Repair Toolbox

• Marked as answer by Basheer Mahdavi Monday, April 27, 2015 11:59 PM

if you are looking for advanced solution for recover Outlook PST file data yopu can download free trial version of datarepairtools from here http://www.datarepairtools.com/outlook-pst-repair.html this tool can fix all PST file issue and make them workable within few mouse clicks .