I’m running Duplicacy 2.1.0 on Windows and have several folders backed up via Symbolic links. Everything seems to be backing up as expected except for certain PST files. The PST files are indeed open via Outlook, but I enabled VSS and was (hoping) that this would allow Duplicacy to workaround the lock file issue.
Is this possible?
I have had frustrations with this problem. The only way I have gotten around it (and it is not a great solution) is by using a pre-run script that kills outlook when the backup starts.
I dont understand why VSS doesn’t handle it and I also dont understand why other backup programs can back up a PST file when it is locked or whatever.
It is a bit annoying… I am just trying to one day get all my users OFF of windows!! 
OR switch to the Thunderbird and be happy. 
I abandoned outlook years ago. With Thunderbird you have not only one large file (PST) (that can be corrupted), but a lot of files, one for each folder in your account, which is much less risky / vulnerable.
The backup works perfectly with it open.
Hopefully @gchen can chime in on this?
I’d love to switch, but have too many programs reliant on Outlook itself as the hub (OneNote tasks and other MSFT products).
As much as I dislike MS products… Outlook actually IMO is still the best email program on Windows. Of course it is an opinion thing but I tried to get people to move to Thunderbird without much luck.
Outside of windows of course… there are many better options
VSS isn’t a foolproof way to open a locked file. It is still up to the program that owns the file to implement the VSS writer interface correctly for it to work. For example, a few programs are known to not work with VSS according to Crashplan (although Outlook wasn’t listed there).
Do you see any related error in the Event Viewer when the file couldn’t be opened?
I personally didnt dig into that much.
From Event Viewer:
Volume Shadow Copy Service error: Unexpected error querying for the IVssWriterCallback interface. hr = 0x80070005, Access is denied.
. This is often caused by incorrect security settings in either the writer or requestor process.
Operation:
Gathering Writer Data
Context:
Writer Class Id: {e8132975-6f93-4464-a53e-1050253ae220}
Writer Name: System Writer
Writer Instance ID: {a6902eb7-cf23-49a6-98ca-836598e50a34}
It also may be possible that you have a filesystem corruption. I often saw all sorts of VSS failures that were magically gone after a pass of chkdsk c: /f
Tried this and nope. Nothing wrong with the filesystem as far as i can tell. I have a separate mirror (and completely different time) of file system and it completes without issue and utilizes vss as it’s backup mechanism.
Actually, CrashPlan claims that they support backing up outlook data files via VSS:
CrashPlan backs up open files like Microsoft Outlook PST files using the Volume Shadow Copy Service (VSS). This approach has a low impact on system resources and is fully supported by Microsoft.
I still think something is screwed up on op’s machine, or perhaps third party antivirus is blocking access. McAffee is known to cause similar issues
No third party virus is causing the blocking as my other backup is capturing the pst just fine (ran at a completely different time to not interfere). i’ve also disabled an uninstalled the second backup mechanism to ensure it was not conflicting too.
As far as i can tell this does appear to be a issue unique to duplicacy.
You can run this command in PowerShell to create a shadow copy:
(gwmi -list win32_shadowcopy).Create('c:\', 'ClientAccessible')
This article has more information: Creating Volume Shadow Copies in PowerShell -- Microsoft Certified Professional Magazine Online
Then you can access the shadow copy by right clicking the C: drive in File Explorer, and then selecting “Previous Versions”. Open the shadow copy just created and a new File Explorer window will appear showing the content of the shadow copy. You can try to copy those .pts files to see if there are any issues accessing them.