Database cannot be upgraded because it is read-only or has read-only files. Make the database or files writeable, and rerun recovery. (Microsoft SQL Server, Error:3415)”
You receive this error when you want to attach the database files to an instance.
You receive this error if the database files that you want to attach to SQL Server Instance (eg mdf, ndf, ldf) are Read-Only. During the attach process, version upgrade may not be performed because the files are in read_only mode.
Make sure that the database files that you are trying to attach are not Read-Only.