Problems Accessing Windows FAT Drives Larger than 2 GB

 Last reviewed: March 18, 1998
Article ID: Q127851
5.x 6.00 6.20 6.21 6.22 95 WINDOWS PUBLIC kbtool kbnetwork kbinterop kbsetup

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The information in this article applies to:

  • Microsoft Windows 98
  • Microsoft Windows 95
  • Microsoft MS-DOS operating system versions 5.x, 6.0, 6.2, 6.21, 6.22

SUMMARY

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

MORE INFORMATION

Both MS-DOS and 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, or 65,536 bytes (where 1K = 1024 bytes). In addition, the maximum cluster size in MS-DOS and Windows 95 is 32K, or 32,768 bytes. Therefore, the maximum logical drive size in MS-DOS and Windows 95 is calculated as follows: 

   32K x 64K = 2048 MB = 2 GB
Windows NT uses a 64K maximum cluster size on 16-bit FAT drives. The maximum logical 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 will still be 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 will be allocated four full clusters. Using a smaller cluster size will typically result 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: 

   TITLE: FAT Type and Cluster Size Depends on Logical Drive Size
   ID: Q67321
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 drive size limit. For more information about the use of large hard disks with MS-DOS and Windows 95, please see the following article in the Microsoft Knowledge Base: 

   TITLE: Windows 95 Support for Large IDE Hard Disks
   ID: Q126855
The FDISK utility in MS-DOS and 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 will not, however, allow the creation of a primary FAT partition or logical drive in an extended FAT 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.

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

NOTE: Microsoft does not recommend using MS-DOS or Windows 95 with a 4 GB FAT drive created in Windows NT. If you use Windows NT's dual boot feature to boot MS-DOS or Windows 95, you should be able to access the drive, but you may experience unexpected behaviors. In particular, some applications or utilities may incorrectly report that 0 bytes of free space exist on the drive.

KBCategory: kbtool kbnetwork kbinterop kbsetup
KBSubcategory: wpp95 win95 diskmem
Additional reference words: 95 msdos
Keywords : diskmem win95 wpp95 win98 kbinterop kbnetwork kbsetup kbtool
Version : 5.x 6.00 6.20 6.21 6.22 95
Platform : WINDOWS

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Last reviewed: March 18, 1998
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