If file-based backup is performed on the database file (mdf / ldf) of Microsoft SQL Database in online state, that file is backed up but it is not valid for restore.

Problem

To back up the Microsoft SQL Server online state database, you must back up via VSS Writer using SQL Server Agent.

However, if you perform a file-based backup on the database file (mdf / ldf), that backup image will also include those files.

When outputting VSS Writer information with the Diskshadow command, database files (mdf · ldf) are enumerated under "SqlServerWriter" as below,

For backing up an online database, it is necessary to back up via VSS writer using "Microsoft SQL Server Agent".

Cause

When file-based backup is performed for volumes including mdf · ldf files, the mdf · ldf files are backed up.

However, since it is not a backup using SQL Agent, the file is not in a consistent state on the database, 
it is impossible to restore the SQL database normally from this file.

Solution

If you do not want to include mdf · ldf files as backup targets during file-based backup, please exclude them manually or use Exclude List.