The following example backup partial the model database to a disk file.

BACKUP DATABASE Syntax:

BACKUP DATABASE { database_name }
READ_WRITE_FILEGROUPS [ , [ ,...n ] ]
TO backup_device [ ,...n ]
[ MIRROR TO clause ]
[ WITH { DIFFERENTIAL | [ ,...n ] } ];

Backup partial backup example:

BACKUP DATABASE model READ_WRITE_FILEGROUPS
TO DISK = 'E:\SQLServerBackup\model_partial.bak'
GO

Backup partial backup with differential example:

BACKUP DATABASE model READ_WRITE_FILEGROUPS
TO DISK = 'E:\SQLServerBackup\model_partial.dif'
WITH DIFFERENTIAL
GO