After upgrading to 1.9, Syncthing for Windows no more support the ‘\\?\UNC\’ path format.
Suppose to have a server ‘servername’ with a share ‘sharename’ with a subdirectory ‘folder\subfolder’ inside.
In Syncthing you would add a folder and specify the path ‘\\?\UNC\servername\sharename\folder\subfolder’.
Syncthing 1.8 is able to succesfully work inside this path and keep the files syncronized but as you upgrade to the 1.9 version you get the following error: 2020-09-11 11:25:23: Error on folder TestUncFolder: folder marker missing (this indicates potential data loss, search docs/forum to get information about how to proceed)
There haven’t been any changes to this on Syncthing’s side since v1.8.0 and I am not aware of any changes on the go side of things. We do use the \\?\ prefix even if you don’t add it yourself to the path, so that’s unlikely to be the cause.
I am testing on a Window 10 x64 version 1909 build 18363.1082; the share in on another Windows 10 x64 version 2004 build 19041.508
The issue, however, is appearing also on Windows server 2008 R2 and on a Windows 7 on a share onto a Qnap Nas.
Please note that I am using the UNC prefix not on a fixed drive but on a network share, hence not a syntax like: \\?\d:\folder\subfolder
(where Syncthing continue to work correctly) but \\?\UNC\servername\sharename\folder
Note the ‘UNC’ part in the path to indicate a network share
Judging by the logfile, Syncthing is simply unable to find the .stfolder folder maker. You do have .stfolder present in the folder, right?
I personally am unable to reproduce this. I have tested with local and network folders using UNC paths, and Syncthing has no problems with recognising them, both under Windows 7 and Windows 10 (v1809 for what it is worth).
Well, all my tests were done with v1.9.0, so there is that.
Can you reproduce the problem using a different network folder, if possible shared from a different machine than the current one?
Also, do you perhaps have any firewall that could come into play here, e.g. blocking the new version of the Syncthing executable from accessing the folder? If you are using Windows Firewall, could you disable it for testing?
Yes: if I use your form, for example: \\servername\sharename\folder
Syncthing functions correctly, is the \\?\UNC\servername\sharename\folder
form that does not work anymore.
Latter form is used and supported by Windows to permit to have filename paths with a lenght more than 250 characters.
The problem happens also with path lenghts less than 40 charecters.
However this issue is strange enough: I see you have tried with the 1.10 release candidate so also I tried it by enabling ‘Upgrade To Pre Releases’ in advanced settings.
With this version the problem does -not- appear anymore; problem disappears also if I upgrade directly from 1.9 to 1.10 rc2.
It seems appearing only with the stable 1.9 version.
Just in case you prefer to stick to stable releases, go to the Advanced Configuration and enable Case Sensitive FS for the affected folder. This will revert to the pre-1.9 behaviour and make the UNC paths work properly. You can deactivate this option later once a stable release of v1.10.0 comes out.