Every Microsoft Windows user has come across an error message saying that a certain operation on a file could not be performed because it was in use, for example while deleting or renaming a file.
The underlying technical reason for this is that the file is open and locked by another process. In the Windows operating system files can be opened exclusively by programs, for example when a program is writing data to a file it wants to make sure no other programs are reading or writing to the same file at the same time. This avoids corruption issues and most Windows programs will lock files they are writing to or modifying. For example, Microsoft Outlook locks the file Outlook.pst while it is running and Windows itself locks a large number of files, e.g. users’ profile files, system logs, etc.
Locked files can cause problems to backup applications such as SyncBack but fortunately there is a built-in feature in Windows that allows accessing locked files. It’s called Volume Shadow Copy Service (in short VSS) and it’s designed to address issues encountered during a backup operation such as copying locked files. VSS provides fast volume capture of the state of a disk volume at one instant in time, a snapshot of the disk volume.
A snapshot is a read-only point-in-time copy of the disk volume which allows the creation of consistent backups of its files, ensuring that the contents do not change and are not locked while the backup is being made. This is very important when copying files that work in tandem and their data are linked or related, for example SQL data files and transaction log files or virtual machine files, because the state of such files is imperative and they need to be copied at that exact state otherwise there may be inconsistencies. Creating a snapshot will inform VSS-aware applications such as MS SQL, SharePoint, MS Exchange, etc., to tidy up their file operations to ensure file consistency prior performing a backup.
On the other hand, SyncBackFree cannot copy locked/open files. You must either exclude those files from your backup or stop any programs that keep the files locked/open prior starting the backup.
We also have a Knowledge Base article explaining how to find which program is using a file.