Once a mass storage device has been properly partitioned, and a file system written to it, the storage is available for general use.
For some operating systems, this is true; as soon as the operating system detects the new mass storage device, it can be formatted by the system administrator and may be accessed immediately with no additional effort.
Other operating systems require an additional step. This step -- often referred to as mounting -- directs the operating system as to how the storage may be accessed. Mounting storage normally is done via a special utility program or command, and requires that the mass storage device (and possibly the partition as well) be explicitly identified.