What’s the issue?
SQL Server backups can be written to several destination types, including local drive letters, UNC paths pointing at network shares, and URLs pointing at object storage such as Azure Blob Storage or S3-compatible endpoints. The destination is specified in the TO DISK or TO URL clause of the backup command.
A backup written to a drive letter lands on storage attached to the SQL Server host itself. That may be a local disk, a LUN presented from a SAN, or a virtual disk, but in each case the backup lives in the same failure domain as the server that produced it.
Why is this a problem?
The backup shares the fate of the server. A host failure, a storage array failure, a ransomware event, or a site loss can take out the database and its backups together, which leaves no recovery path at exactly the moment one is needed.
Recovery to a different server is also slower. Restoring elsewhere requires first retrieving the file from the original host, which may be unavailable, and the copy step adds time to an outage that is already underway.
Local backups are frequently only the first stage of the strategy, with a separate process copying them to another location afterward. That is a reasonable pattern, but it means the actual protection depends on the copying completing successfully, and a failure of this process may not be monitored if the SQL Server backup itself succeeded.
What should you do about this?
Review recent backup history in msdb.dbo.backupmediafamily to identify which databases write to local drives and which process is taking those backups. Determine whether a secondary copy process exists, and confirm it is running and monitored.
Where a copying/sweep process is the design, verify how quickly files are copied off the host, how failures are detected, and how long a backup remains only on local storage. Also verify alerts are in place and functioning as expected.
Where no secondary copy exists, change the backup destination to a network share or object storage so the backup lands outside the host from the start.