When you attempt to defragment an NTFS partition, you receive a message similar to:

Disk Defragmenter cannot defragment NTFS drives with more than 4 KB per cluster.

This error occurs because the NTFS compressed file functions map files in memory by using 4 KB blocks, so cluster sizes (allocation units) above 4 KB are NOT supported.

If you wish to defragment, or compress, you must format the partition with an allocation unit size of 4096 (4 KB) or less.

NOTE See Disk Defragmenter limitations in Windows 2000, Windows XP, and Windows Server 2003.