as a follow up to the post that I just submitted,
I should point out that the device name confusingly has the same name as the database name itself so the command
BACKUP DATABASE sample_db TO sample_db WITH DIFFERENTIAL
go
means backup the database sample_db to the device sample_db as listed in sp_helpdevice.
sp_helpdevice
DEVICE_NAME PHYSICAL_NAME
sample_db s:\backups\sample_db.dat
sample_dbLog s:\backups\sample_dbLog.dat