You cannot do backup to a mapped network drive, you have explicitally give the share location. Refer the below example.
\\ServerName\ShareFolderName\
RESTORE DATABASE MyDb
FROM Disk = '\\serverA\Mssql\Backup\Mydb.bak'
WITH STANDBY = 'C:\MSSQL\Log\undoMyDb.ldf', MOVE 'MyDb_Data' TO 'C:\MSSQL\Data\MyDb_data.mdf',
MOVE 'MyDb_log' TO 'C:\MSSQL\Log\MyDb_log.ldf'
GO
Try this
Dr.Sql
Good Luck.