On a standard Windows PC a cluster on a hard drive is comprised of 8 sectors. It is the smallest unit that a file system can address.
The word standard is emphasized here, as there are numerous variations, e.g a RAID attached to a Windows server could have a very different cluster settings. But for for a standard Win 2000/XP with an NTFS file system there is a single hard, the standard is 8 sectors, which is 4 KB.
This means that for a file that is “non-resident data” the smallest physical size it will take up on a hard drive will be 4 KB.
For example if a file is just 2048 bytes long, e.g a text file with 2048 characters, then the file will still be given 4 KB (4096 bytes) of space, as this is 1 cluster and this is the smallest unit that the file system can handle.
Equally if file is 5000 bytes long (i.e just over cluster) it will be allocated 2 clusters – 8 KB.
The remaining space – between the end of the logical file, and the end of the physical space given, e.g between the 2048th btye and the end of 1 cluster, or between the 500th byte and the end of the second cluster is known as “slack”.