It is not part of a multi-family media set. You can use "bakup with format" to construct a new media set.

The volume on device c: \ Program Files \ Microsoft SQL Server \ MSSQL \ data is not part of a multi-family media set. You can use "bakup with format" to construct a new media set.

Why can't I back up the database?

It is because you have created a media set before, that is, several backup devices are combined as a whole backup device. therefore, during this backup, you may only specify one of the backup devices, so the above error occurs. the solution is to write all the backup devices.

Backup database test to disk = 'e:/bakup/1. Bak', disk = 'e:/bakup/2. Bak'

My solution is ..

Run
Backup database dbname to disk = 'e:/bakup/1. Bak'

Backed up .......