SAN replication has VSS support in Windows that is triggering the SQL Server VSS Writer at backup time. The VSS Writer is effectively creating a snapshot on the database and running a full backup which you probably see logged in msdb database. Try to implement full backups daily and take transaction log backups every 15 mins if possible. If you disable VSS writer you might see transactional integrity issues on the SAN copy. Please double check. If you are not worried about the SAN snapshots then it's good to disable the VSS writer.