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Clusters - Allocation Unit:

The smallest amount of disk space allocated to hold a file. All File Systems used by WinXP Pro & Win2000 organize Hard Disk Drives (HD Drives) based on Cluster size. Cluster and allocation unit are synonmous. Cluster size is determined by the number of Sectors the Cluster contains. A Sector is the smallest unit of storage read or written on a HD Drive.

The smaller the Cluster size, the more efficiently a Disk stores information. If no Cluster size is specified during formatting, Win2000 chooses Defaults based on the size of the Volume. These Defaults are selected to reduce the amount of space lost and the amount of Fragmentation on the Volume. Allocation unit size is also called cluster size. Defrag often!

A comparison of FAT16, FAT32, and NTFS Volume and default Cluster Sizes for Volumes with WinXP Pro File Systems. See: Disk Management.

¹Windows XP Professional formats FAT32 volumes up to 32 GB regardless of cluster size. To format volumes larger than 32 GB, you must use NTFS. However, Windows XP Professional can mount FAT32 volumes larger than 32 GB that were created by other operating systems.

In the Disk Management snap-in, you can specify a cluster size of up to 64 KB when you format a volume. If you use the format command to format a volume, but do not specify a cluster size by using the /a:size parameter, the default values in Table 13.4 are used. If you want to change the cluster size after the volume is formatted, you must reformat the volume.

Before choosing a Cluster size other than Default, note these important limitations:

1. For WinNT, Win2000, and WinXP Pro, Cluster size of FAT16 volumes from 2 GB through 4 GB is 64 KB, which can create compatibility issues with some applications. For example, setup programs do not compute free space properly on a volume with 64-KB clusters and cannot run because of a perceived lack of free space. For this reason, you must use either NTFS or FAT32 to format volumes larger than 2 GB. The format command in WinXP Pro displays a warning and asks for a confirmation before formatting a Volume that has 64-KB clusters using FAT16.
2. WinXP Pro, like WinNT 4.0 and Win2000, supports file compression. Because File Compression is not supported on Cluster sizes above 4 KB, the default NTFS Cluster size for WinXP Pro never exceeds 4 KB.

To verify the Cluster size of an existing Volume: From Command Prompt - Use chkdsk command or fsutil fsinfo ntfsinfo commands.

References:

1. www.microsoft.com/technet - Cluster Size:
2. msdn.microsoft.com - Clusters and Extents: "Clusters may be referred to from two different perspectives: within the file and on the volume. Any cluster in a file has a virtual cluster number (VCN), which is its relative offset from the beginning of the file. For example, a seek to twice the size of a cluster, followed by a read, will return data beginning at the third VCN. A logical cluster number (LCN) describes the offset of a cluster from some arbitrary point within the volume. LCNs should be treated only as ordinal, or relative, numbers. There is no guaranteed mapping of logical clusters to physical hard drive sectors."
   1. Virtual Cluster - One of the clusters of a file for which the clusters are in the order in which they are read sequentially in the file. Virtual clusters are distinguished from logical clusters.
   2. Logical Cluster - One of the clusters of a file for which the clusters are in the order in which they are found on the hard drive. Logical clusters are distinguished from Virtual Clusters.
   3. HD Disk Defragmentation:
3. www.microsoft.com/technet - New in File Systems:
4. 192322 - Description of Default Cluster Sizes for FAT32 File System: Win9x Versions
5. 184006 - Limitations of FAT32 File System: FAT32
6. 154997 - Description of the FAT32 File System:
7. Microsoft Windows XP - New in File Systems:
8. 140365 - Default Cluster Size for FAT and NTFS:
9. 314878 - The Default Cluster Size for the NTFS and FAT File System:
10. Microsoft Windows XP - Advantages of Using NTFS: