What is Cluster?
A group of disk sectors. An operating system doesn't access the disk per sector, it accesses the disk per cluster.
The operating system assigns a unique number to each cluster and
then keeps track of files according to which clusters they use. Occasionally, the operating system
marks a cluster as being used even though it is not assigned to any file.
This is called a lost cluster. You can free up disk space by reassigning lost clusters,
but you should first make sure that the clusters do not, in fact, contain valuable data.
In DOS and Windows, you can find lost clusters with the ScanDisk utility.
DOS and Windows keep track of clusters with the File Allocation Table (FAT). The size of each cluster
depends on the disk's partition size.
What is Root cluster?
The cluster in the partition that contains the start of the root directory.
Only applicable to FAT32 partitions. The location of the root cluster is needed to rebuild a damaged boot sector.