Error - Operating system error 5(Access is denied).
You cannot back up databases to a network drive if your account has not sufficient permissions to access the network drive. The "Operating system error 5" error is related to security.
Copy the backup file (.bak) file to a folder where all permission is set and then try. It will work.
You will get the same type of error when you want to restore a database from/to a Compressed Folders (which looks blue in colour).