When you create a database, by default the transaction log is put on the same device and in the same directory as the database. This arrangement does not protect against all kinds of media failure, and you should consider placing the transaction log in another location for production use.
For comprehensive protection against media failure, you should keep the transaction log on a different device from the database file. Some computers with two or more hard drives have only one physical disk drive with several logical drives or partitions: if you want reliable protection against media failure, make sure that you have a computer with at least two physical storage devices.
Placing the transaction log on a separate device can also result in improved performance by eliminating the need for disk head movement between the transaction log and the main database file.
You should not place the transaction log on a network directory. Reading and writing pages over a network gives poor performance and may result in file corruption.