Yes, this is supported but still not recommended. Performance concern While there are no reading/writing issues similar to connecting to pst-files on a network share (which!), external drives are often slower and/or have power saving features which could interrupt or slowdown Outlook performance. With USB 3.0 hard disks, the performance issue is basically gone. For USB-sticks (or flash-drives), this isn’t always the case as they usually struggle more with multiple read/write operations at the same time. Close Outlook before detaching Another (and even bigger!) concern is that Outlook requires continuous access to a pst-file. When Outlook loses the connection to a pst-files while Outlook is still running, the pst-file could get damaged. In order to prevent this, you must always remember to first close Outlook before shutting down or detaching your external drive.
If you forget to do this, you can use to check your pst-file for errors. Detach pst-file after every use When you connect to a pst-file, Outlook will add a reference to this pst-file to your mail profile. The next time you start Outlook it will automatically try to open the pst-file again. If you would open Outlook when the file is missing, because the external drive is detached, you could end up with several error notifications about the missing pst-file or even end up. To prevent this from happening, it is recommended that you detach the pst-file from the Outlook mail profile after each use. You can do this via; • Outlook 2007 and previous File-> Data File Management-> select pst-file-> Remove• Outlook 2010 and Outlook 2013 File-> Account Settings-> Account Settings-> tab Data Files-> select pst-file-> Remove Note: The lock on the pst-file will still be kept.