You are correct that SQL Server databases are saved locally first. The reason is that with SQL Server, we use the native backup features to perform the backup and then send the final backup to the desired backup storage. You can change the temp / default location for these SQL Server backups by adjusting the value in the Options dialog (system menu) on the Advanced tab - Temporary Folder option.

If you turn on Compression, then SQL Server compresses the database first, but actual disk space used depends entirely on the database size, log file size (if any), and how compressible the database is. Many databases can compress 80% or more, but some, if they contain a lot of binary data or use table/row compression or TDE, will compress less. Differential and transaction log backups will vary in size based on actual data changes, so estimations for those backups types are not possible. Since we do not know how well the database will compress before backup, we cannot guess if the backup will fit.

As you stated, you have other disks, so select one where the database files are not located for best performance. As an example, if your database files are on E, then consider using F for backups.

Thanks!

Thanks! I've reconfigured the temp folder location to use a folder on F:, and will see how well that works when the scheduled job runs next.