Quoting from this link:http://cquirke.mvps.org/9x/fat.htm
Cluster: Single unit of data storage at the FATxx file system logic level
Sector: Single unit of storage at the physical disk level
A cluster is the smallest unit of storage the operating system's file system can manage. For example, if a 1KB file is stored in a system with a 32KB cluster size (see table below), the 1KB file takes up 32KB of disk space. Below are the cluster sizes for the FAT32 file systems on Windows computers.
Disk Size Cluster size
512MB-8GB 4KB
8-16GB 8KB
16-32GB 16KB
32GB+ 32KB