The organization is called a filesystem. Each different filesystem provides the host operating system with metadata so that it knows how to read and write data. When the medium (or media, when the filesystem is a volume filesystem as in RAID arrays) is mounted, these metadata are read by the operating system so that it can use the storage.
The organization is referred to as a filesystem. Each distinct filesystem supplies the host operating system with metadata, allowing it to understand how to read and write data. When the medium (or media in RAID arrays) is mounted, these metadata are accessed by the operating system for utilizing the storage.