To increase the amount of storage space allocated for the transaction log when you have used the log on extension to create database, include the name of the log’s device in the log on clause when you issue alter database.
If you did not use the log on extension of create database to place your logs on a separate device, you may not be able to recover fully in case of a hard disk failure. In this case, you can extend your logs by using alter database with the log on clause, then using sp_logdevice to move the log to its own devices.