Outlook OSTs in both AppData and data drive

[jmt356]

I moved my Outlook PSTs and OSTs to my data drive. However, following an upgrade to Office 2016, I have found OSTs in AppData\Local\Microsoft\Outlook.

One of my OSTs exists in both AppData\Local\Microsoft\Outlook and my data drive with the same name (username.ost), but the one in AppData is 4 GB and the one on my data drive is 6 GB. How do I know which of them is the one that Outlook 2016 is reading? They both show Date modified as today, so it would seem they are both syncing with the Exchange server, though the one on my data drive is significantly bigger.

As for my other exchange account, in AppData, it exists as two separate files—emailaccount@example.org.nst (16 MB) and organization.ost (300 MB). On the data drive, it exists as emailaccount@example.org.ost (500 MB). Which of these is actually being read by Outlook? They all show up as updated as of today, so it would seem they are all syncing with the Exchange server, but there are significant size differences between the files which implies not all of them have all the data.

[JoeP]

In Outlook, go to File | Office Account | Account Settings | Account Settings. Click on the Data Files tab. That will show you which files Outlook is using for data storage.

See Introduction to Outlook Data Files (.pst and .ost) for information about Outlook file types.

[jmt356]

Account Settings | Data Files shows the files in AppData as those being used for data storage. If that's the case, I wonder what the quasi-duplicate files on my data drive are. They are bigger than those in AppData and have similar names (username.ost is in both AppData and the data drive and emailaccount@example.org.nst is on AppData and emailaccount@example.org.ost is on the data drive).

Also, how can I determine whether the files in AppData or the files on the data drive are the files that are being used for indexing? If the files in data drive are being used for indexing but Outlook is using the files in AppData for data storage, then that would explain my my searches in Outlook turn out with no results.

[StuartR]

Did you try rebuilding the index?

[jmt356]

I started rebuilding it about 15 hours ago. About 200,000 items have been indexed so far. I believe there are about 300,000 items left to go.

[jmt356]

On 6 Nov., the index rebuild was complete. Search has been working.

I am still researching why there are OST files in both AppData in C as well as on the Data D drive.

[jmt356]

I discovered the underlying issue. I mistakenly believed that duplicate OSTs for my Exchange accounts existed in AppData and on my D drive. It turns out that is not the case. They are not duplicates. The OSTs in D were those that existed prior to my upgrade to Office 2016. When I upgraded to Office 2016, new OST files for my Exchange accounts were created in AppData. Now, when I launch Outlook 2016, it reads from and updates the OSTs in AppData. However, when I launch Outlook 2010, it reads from and updates the OSTs in D.

When I try pointing Outlook 2010 to the OSTs in AppData, Outlook 2010 cannot read them. It states that they were created with a newer version of Outlook. This makes little sense to me; the files in AppData and in D appear to be identical to one another, and they all use the same .ost extension.

It seems that for now, I need to maintain separate OST files for Outlook 2010 and Outlook 2016. If I ever discontinue using Outlook 2010, I can delete the Outlook 2010 OST files in D.

[JoeP]

You can not see the internal structures of the files. Because Microsoft does not expect that the .OST files will be used outside of Outlook they feel free to change data structures between versions or add new structures to support some new features. An older version of Outlook does not know the new internal structure and therefore can't properly process the newer version file. So, you must use a version of Outlook which is compatible with the version of the .OST file.