# Hard Drive Sector format question.

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December 8, 2006 10:44:53 PM

A hard drive has four platters. It has been low-level formatted to contain 64 sectors and 64000 tracks. If a single FAT32 partition is created, the size of a file allocation unit is exactly ________ bytes
December 9, 2006 1:55:39 AM

FAT32 partitions theoretically have 4 bytes reserved for the cluster number (allocation unit number), but there is another restriction. The top 4 bits of that number are reserved, leaving 28 bits. So the theoretical maximum number of clusters on a FAT32 volume is 2^28 = 268,435,456.

So, for your theoretical drive, the number of sectors on the drive is as follows:

4 platters = 8 heads * 64,000 cylinders * 64 sectors/track = 32,768,000 sectors on the drive.

Theoretically, this is less than the maximum cluster number for the volume, so you could make the cluster size equal to the sector size (512 bytes per cluster). However, this in turn makes the file allocation table itself very large (4 bytes per entry, 32,768,000 entries = 131MB just for the FAT). So, what Windows typically does is limit the FAT size to 2^21 entries/clusters (2,097,152 * 4 bytes = 8 MB) unless the drive size is so large that it becomes necessary to exceed that.

Thus, the 32,768,000 sectors on your drive will grouped into clusters such that the maximum cluster number is <= 2,097,152. 32,768,000/2,097,152 = 15.625. Round that up to the nearest power of 2 = 16 = 2^4. 16 * 512 bytes per sector = 8,192 bytes = 8K cluster size.

If your drive were to exceed 64GB (which would give a 32K cluster size), then Windows will allow the cluster numbers to exceed 2,097,152, up to the maximum of 268,435,456 clusters, which at 32K per cluster is an 8 TB volume size. Of course, at 2TB you exceed the partition mapping capabilities of the standard MBR, so you need an advanced partitioning scheme like GPT disks, which aren't supported except by Windows 2003 Server SP1.
December 9, 2006 1:08:46 PM

Start,
Run,
MS-ITS:C:\WINDOWS\Help\gen.chm::/guid_partition_table.htm

Does FAT32 support 64 KB clusters ?
(I been running NTFS too long, and have hit the 2.2 (apx) TB limit once with NTFS, never tried with FAT32 - that is just crazy, NTFS will do it with 4 KB or 8 KB clusters, and a better MFT system).
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December 9, 2006 4:10:44 PM

wait i got a little confused here.. so what is the actual size of the file allocation unit in bytes ... which of these answers?

i see cluster size and sectors..
December 9, 2006 11:03:45 PM

File allocation unit size is the same thing (just a different wording) as the cluster size.

As I showed above, when Windows formats the FAT32 partition, your file allocation unit (cluster) size on your specified hard drive will default to 8192 bytes (8 KiB).
!