Microsoft KB Archive/127851

= Problems Accessing FAT16 Drives Larger Than 2 GB =

PSS ID Number: 127851

Article Last Modified on 5/6/2003

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The information in this article applies to:


 * Microsoft Windows 98
 * Microsoft Windows 95
 * Microsoft MS-DOS operating system
 * Microsoft Windows NT Workstation 3.5
 * Microsoft Windows NT Workstation 3.51
 * Microsoft Windows NT Workstation 4.0
 * Microsoft Windows NT Server 3.5
 * Microsoft Windows NT Server 3.51
 * Microsoft Windows NT Server 4.0
 * Microsoft Windows Millennium Edition

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This article was previously published under Q127851



SUMMARY
Microsoft Windows NT supports the creation of primary partitions and logical drives of up to 4 gigabytes (GB) using the File Allocation Table 16 (FAT16) file system, but Windows 95/98 and MS-DOS do not support these drives. The size limit for logical drives using the FAT16 file system in Windows 95/98 and MS-DOS is 2 GB.

NOTE: Windows 95 OEM Service Release 2 and later support drives larger than 2 GB using the FAT32 file system. For more information about FAT32, please see the following article in the Microsoft Knowledge Base:

ARTICLE-ID: 154997

TITLE : Description of the FAT32 File System

Windows NT 4.0 does not support FAT32, but FAT32 support has been added in Windows 2000.



MORE INFORMATION
Microsoft does not recommend using MS-DOS or Windows 95/98 with a 4 GB FAT16 drive created in Windows NT. If you use Windows NT's dual boot feature to boot MS-DOS or Windows 95/98, you may be able to access the drive, but you may experience unexpected behavior. In particular, some programs or utilities may incorrectly report that no free space (0 bytes) exists on the drive.

In addition, you cannot run Windows 95/98 Setup if your computer has a FAT16 drive created by Windows NT.

For example, during Windows 98 Setup you receive the following error message if your system has a FAT16 drive larger than 2 GB:

Setup has detected that your hard drive has a 64K-cluster FAT partition. Because ScanDisk does not work on disks with this cluster size, Setup cannot continue. To complete Setup, you must repartition your hard drive, format the partition with a FAT file system that has a cluster size of 32K or less, and then restart Setup.

Other problems accessing FAT16 drives larger than 2 GB in Windows 95/98 may include the following:


 * Running the Defrag tool on the drive causes a "Divide by zero" error message or the Defrag tool stops responding (hangs).


 * ScanDisk for MS-DOS stops responding (hangs) or causes an "Out of memory" error message.


 * The Chkdsk tool may report multiple "allocation errors" on the drive.

ScanDisk for Windows typically runs without error on 64k cluster FAT16 drives and shows 64K clusters/allocation units on the Summary dialog.

Both MS-DOS and the retail release of Windows 95 use a 16-bit FAT for logical drives larger than 15 megabytes (MB). The maximum number of clusters for a 16-bit FAT drive is 64K. In addition, the maximum cluster size for a FAT16 drive in MS-DOS, or Windows 95 is 32K. Therefore, the maximum logical drive size for FAT16 is calculated as follows:   32K x 64K = 2048 MB = 2 GB The maximum cluster size for a FAT16 drive in Windows 98 is 32K, or 32,768 bytes. Therefore, the maximum logical drive size for FAT16 is calculated as follows:   32K x 64K = 2048 MB = 2 GB Windows NT uses a 64K maximum cluster size to allow drives larger than 2 GB using FAT16. The maximum logical FAT16 drive size in Windows NT is calculated as follows:   64K x 64K = 4096 MB = 4 GB

NOTE: On a FAT drive, space is allocated by clusters. That is, a file that is smaller than the drive's cluster size is still allocated one full cluster, thus wasting the additional space in the cluster. Similarly, a file that is large enough to fill 3-1/2 clusters is allocated four full clusters. Using a smaller cluster size typically results in less wasted space on the drive.

The cluster size for a FAT drive is determined when the drive is formatted and varies depending on the size of the logical drive. For more information about cluster and logical drive sizes in MS-DOS, please see the following article in the Microsoft Knowledge Base:

ARTICLE-ID: Q67321 67321

TITLE : FAT Type and Cluster Size Depends on Logical Drive Size

Windows 95 and MS-DOS use the FDISK utility to partition a disk. When it accesses a hard disk, FDISK uses the system AT ROM BIOS INT13h interface, which has a maximum of 1024 cylinders, 255 heads, and 63 sectors per track. FDISK can access any drive within the limits imposed by the AT ROM BIOS, which means that FDISK can access drives of up to 8 GB, calculated as follows:

  1024 cylinders x 255 heads x 63 sectors per track x 512 bytes per sector = 8,422,686,720 bytes, or roughly 8 GB

The original IDE hardware interface is limited to 16 heads, which reduces the maximum drive size to 504 MB. Newer IDE (ATAPI) technology, however, uses a translation scheme called Logical Block Addressing (LBA) to exceed the 504 MB limit as imposed by the system AT ROM BIOS and IDE specifi- cation. SCSI and ESDI hard drive controllers use similar translation methods that are usually built into the controller card's ROM BIOS to exceed the 504 MB size limit. For more information about the use of large hard disks with MS-DOS and Windows 95/98, please see the following article in the Microsoft Knowledge Base:

ARTICLE-ID: 126855

TITLE : Windows Support for Large IDE Hard Disks

The FDISK utility in MS-DOS and the retail release of Windows 95 can create an extended partition larger than 2 GB. FDISK can then create multiple formatted drives of up to 2 GB in this extended partition that conform to the AT ROM BIOS constraints mentioned earlier. FDISK does not, however, allow the creation of a primary FAT16 partition or logical FAT16 drives in an extended partition that is larger than 2 GB. Note also that, as mentioned earlier, logical drives larger than 504 MB cannot be accessed using MS-DOS or Windows 95 unless LBA or geometry translation is used.

In Windows NT, you can create 4 GB FAT16 drives, either as a primary partition or as a logical drive in an extended partition. Because MS-DOS and Windows 95/98 have a maximum logical FAT16 drive size of 2 GB, FAT16 drives larger than 2 GB created using Windows NT cannot be reliably accessed using MS-DOS or Windows 95/98.

Additional query words: 98 msdos

Keywords: kbDiskMemory kbinfo kbinterop kbsetup KB127851

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