The space in a hard drive is divided into units called sectors, usually of 512 bytes. The filesys¬tem groups the sectors together after creating a sector index; the group is called a cluster or an allocation unit.

It's difficult for the filesystem to deal with data on a sector-by-sector basis, so when the filesystem needs to access a sector for a read-write operation, it will first define the cluster number of that sector. Then from that cluster, it will use the sector index to access that particular sector.

Each logical volume (hard drive) has a table-called the File Allocation Table (FAT)-that carries all the information about the sectors and the files stored on those sectors.

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