4096 bytes
When you use the Convert.exe utility to convert a FAT partition to NTFS, Windows always uses the original FAT cluster size as the NTFS cluster size for cluster sizes up to 4 KB. If the FAT cluster size is greater than 4 KB, then the clusters are converted down to 4 KB in NTFS. A single NTFS volume of 16 TB is quite large, but there are use cases for drives this large. The issue is that the minimum allocation unit goes from 4K to 8K when the NTFS volume exceeds 16 TB. There is another threshold at the 32 TB level and more as the file system scales up. A hard disk partition (also known as a volume) can be formatted to NTFS, FAT, or exFAT. The default values are used by Windows when one of the following methods is used to format the partition: By default, the maximum cluster size for NTFS under Windows NT 4.0 and later versions of Windows is 4 kilobytes (KB). Jonathan writes: If you are a “Standard User” by Microsoft’s definition, you should keep the default 4096 bytes. Basically, the allocation unit size is the block size on your hard drive when it formats NTFS. All Windows Server administrators have done this: quickly format an NTFS volume and select the default allocation unit size without giving it much thought. It turns out the unit size is quite important when it comes to the layout and practicality of the drive. I do this for non-boot volumes. Figure A. Windows is adaptive in the size of the volume and displaying the available allocation unit sizes for a volume based on its size. Consider this example with a single drive that was initially 13 TB and then expanded to 19 TB. The larger the allocation unit size, the faster the drive will perform. The smaller the allocation size, the less wasted space will be used by small files. With a 1TB HDD, typically you should set it to max size (4096kb I believe). I have never changed any of the drives from the default size in years.... The following table describes the default cluster sizes for exFAT. 1  Using the FORMAT command from the command line without specifying a cluster size. 2  Formatting a volume from Windows Explorer when the Allocation Unit box in the Format dialog box lists Default Allocation Size.
https://support.microsoft.com/en-us/kb/140365 http://www.techrepublic.com/blog/the-enterprise-cloud/ntfs-allocation-unit-sizes-for-large-volumes/ https://support.microsoft.com/en-us/kb/140365 http://www.howtogeek.com/136078/what-should-i-set-the-allocation-unit-size-to-when-formatting/ http://www.techrepublic.com/blog/the-enterprise-cloud/ntfs-allocation-unit-sizes-for-large-volumes/ http://www.techrepublic.com/blog/the-enterprise-cloud/ntfs-allocation-unit-sizes-for-large-volumes/ http://www.tomshardware.com/answers/id-1659864/allocation-unit-size-1tb-black.html https://support.microsoft.com/en-us/kb/140365
what is best allocation unit size for ntfs