You've already read that there are multiple sectors on each track in the disk, and you know that a single sector usually can store 512 bytes of data. That means that bigger files must be placed in a lot of sectors, depending of the size of the file. Consider having a large amount of data and a lot of files. That means that a single file would be placed in a lot of sectors. Managing so many sectors can be a very hard and time-taking job for the HDD. Therefore, FAT format, uses clusters, or also known as allocation units. Clusters are nothing more then grouped sectors into a bigger storage place. Clusters can contain 4 sectors, or even 64 sectors. So, instead of working of a great number of individual sectors, the HDD now works with a greatly reduced number of clusters. The size of the cluster has a great influence on the disk performance and utilization.
We have already mentioned, in the Device section, that HDD is a block device. That means that data is stored and arranged in blocks.