Hope you are trying to restore the backup file that was backed up from another system. There could be a possiblity of drive mismatch between the two servers.
Eg: Source server from where you backed up the DB call it as ServerA, would have the db files stored in drive E. But the destination server call it as ServerB, would have drive E with lesser space OR assigned to CD drive. So in that case while restoring you may get this error.
Check for that. If so, you can restore the database like this with a WITH MOVE option in Sql Query Analyser and specify the drives that you have enough space and want it to be restored to.
RESTORE DATABASE TestDB
FROM DISK = 'c:\Northwind.bak'
WITH MOVE 'Northwind' TO 'c:\test\testdb.mdf',
MOVE 'Northwind_log' TO 'c:\test\testdb.ldf'
Hope that explains.
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