If you’re getting “The system cannot find the file specified” errors from Arq on Windows, it may be due to the files not being included in VSS snapshots.
Arq uses Windows Volume Shadow Copy Service (VSS) to back up files that are open/locked. A common example is Outlook .pst and .ost files. While Arq will back up the .pst file, by default it will fail to back up the .ost file. Arq might log errors like this:
C:\Users\stefan\AppData\Local\Microsoft\Outlook\firstname.lastname@example.org: (2) The system cannot find the file specified: [\\?\GLOBALROOT\Device\HarddiskVolumeShadowCopy47\Users\stefan\AppData\Local\Microsoft\Outlook\email@example.com]
This is because it’s excluded from VSS snapshots by default. A Microsoft TechNet blog post explains why:
Maintaining changes to .ost files within shadow copies is expensive in terms of space and I/O activity. The performance impact doesn’t occur during the image backup itself–the only extra work at backup time is backing up the .ost file as part of the image. Instead, the performance impact occurs during the ongoing, everyday I/O to the .ost file when Outlook is running. If the .ost changes were kept in shadow copies, then every time Outlook writes to the .ost file, the result is a copy-on-write I/O hit (2 writes, 1 read). Although we have worked to reduce the impact of copy-on-writes on shadow copies, a heavily churned file like an .ost file could still cause problems. For these reasons, and the fact that .ost files can be regenerated, we chose to delete .ost files from the shadow copy before the image is created.
You can change the registry to remove the exclusion. A separate Microsoft blog post explains how to modify the registry to exclude files: Excluding Files from Shadow Copies
But Microsoft says that since the .ost file can be regenerated, there’s no point in backing it up. So the better choice is probably to exclude it. Click on your folder on the left in Arq, then click the “Edit Backup Selections” button. Navigate to your Outlook data folder (usually something like C:\Users\<user>\AppData\Local\Microsoft\Outlook), uncheck the .ost file, and then click Save.
Many thanks to David Middleton for finding this solution to the problem!