Why does moving Documents to OneDrive (a so-called “Backup”) stop Outlook from working with PST files and what can be done to fix it? It’s a bug in Outlook (classic) but easily fixed.
This is a long-standing bug in Outlook (classic) which Microsoft shows no interest in acknowledging, let alone fixing. The fix is simple but not obvious and should not be necessary at all. Outlook (new) for Windows is completely different software so none of this applies.
See Why have my files moved to OneDrive? to understand what Windows does when it offers a “Backup” or moving key folders to OneDrive.
PST vs OST in Outlook (classic)
Most modern mailboxes synchronize data between online and Outlook. Classic Outlook uses OST files to keep a local copy of the information primarily saved online. OSTs are the automatic choice for all Microsoft hosted email (including Outlook.com) plus other IMAP connections, most notably Gmail.
PST files are where Outlook (classic) saves email and other information moved from mailboxes connected by the older POP system. This is still common for email hosted by ISP’s (Internet Service Providers)
PST files should always be backed up because they are often the only copy of those message, contacts etc.
OST files don’t need backups as much because they are already copies of the main online store.
Like many things with Outlook (classic), there are exceptions. We’ve seen PST files used for mail connections because the connection settings haven’t changed for many years, despite the arrival of newer protocols. Or users have chosen POP (therefore PST storage) for some, perhaps forgotten, reason.
How to tell OST from PST storage in Outlook classic
Go to File | Account | Account Settings | Data Files then look at the file extension in the Location column. This also shows where the OST or PST is saved.
Where are Outlook data files stored?
Outlook data files (OST or PST) can be saved anywhere, but they are best on the fastest drive available, usually a SSD.
OST files are placed at \users\<user name>\AppData\Local\Microsoft\Outlook\
PST are usually in the \My Documents\Outlook Files\
That’s important because “My Documents” is one of the Known Folders that Microsoft will offer to move to OneDrive, under the guise of “Backup’.
When Outlook PST files move to OneDrive
If the Documents folders are moved (aka “Backup”) to OneDrive that triggers a bug in Outlook (classic). Microsoft does think it’s a bug but it really is.
Most PST files are usually in the “\Documents\Outlook files\” folder.
If the My Documents folders are switched to OneDrive (what Microsoft calls “Backup” Instead of being at \Users\<user name>\Documents\Outlook files\ any PSTs are now at
\Users\<user name>\OneDrive\Documents\Outlook files\
When Outlook next starts, it can’t find any PSTs because the file has moved to a different folder and Microsoft hasn’t automatically adjusted Outlook settings. Outlook will show an error message like this:
Click OK and a File Open dialog appears. Select the new location for the PST and Outlook should sort itself out.
It’s a bug because Outlook (classic) should automatically handle the change of PST location, just like other Office apps do. That’s why you don’t have the same problem with Word, Excel or PowerPoint.
Outlook should be using the ‘Known Folder’ technique to find the “Documents” location, not a specific path. Again, that’s what the other Office/Microsoft 365 apps do.
If that’s too hard, at least some code in Outlook to relocate the PST automatically. After all, the change in PST location is done by Microsoft so it’s the company’s responsibility to fix it in their own programs.
