Q. What is High level formatting?
In any disk system the space at some time in use would become discarded and therefore will be free for other application. Operating system assigns disk space on demand by user programs. Normally space is allocated in units of fixed size known as an allocation unit or a cluster that is a simple multiple of disk physical sector size typically 512 bytes. DOS operating system forms a cluster by joining two or more sectors so that smallest unit of data access from a disk turns into a cluster not a sector. Usually size of the cluster can vary from 2 to 64 sectors per cluster.
High level formatting includes writing directory structures and a map of free as well as allocated space (FAT or INODE) to disk. Generally this also means transferring boot file for operating system on hard disks.