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FDisk is probably the original Hard Drive partitioning utility, I believe it has been around as long as PC Hard Drive's have. It is a DOS command processor that supports basic functions for the Microsoft (and compatible) world only. Since its use was intended to be with DOS and the Win9x line it does not provide support for NTFS partitions.

FDisk has some major drawbacks so it is not overly popular anymore. Many find the DOS character-based interface clumsy and difficult to use. It provides only the minimum required functionality, primarily restricted to creating and deleting two basic partition types, Primary and DOS Extended - strangely enough, the only ones supported by Microsoft products. This also implies that its use is destructive to all data in any partition you make changes to. There is no support for resizing or moving a partition - well, short of the brute force method of deleting and re-defining the partitions with the new sizes. FDisk can also only display and work with one drive at a time. If you have more than one physical drive, you must insure that you have selected the correct one (disk 1, disk 2 etc.) before making any changes.

FDisk Operation

If you're going to use FDisk, make sure you have an appropriate version, I'd suggest nothing earlier than the latest 98SE version which corrected support for physical disks over 64GB (although it is still limited to a maximum of 137GB). A user reported that he had success even with a 300GB drive. My Laptop Boot Disk and Desktop Boot Disk include the latest version of FDisk. Early versions of FDisk only had support for FAT16 partitions which were limited to 2GB in size and generally did not support physical drives larger than 8GB.

Boot to the floppy and at the A:\> prompt type fdisk and press Enter on your keyboard.

When first started FDisk should display the Large Disk Support warning:

You definitely do want to select Yes for this support as long as your hard drive is larger than 512MB. Selecting Yes will result in the drive being setup for FAT32 support. Selecting No will result in the drive being setup for FAT16 support. Following that screen you will be presented with the Main Menu:

Option 5 is the one to use to get to the Disk Selection Menu where you can select which disk you want to work with:

For a new disk you would choose option 1, Create DOS Partition or Logical DOS Drive from the main menu:

Once you select which type of partition to create you will be asked to provide information on the size of the partition.

Note: Hard drive manufacturers calculate the size of their drives using binary numbers. So, for example, a 80GB drive will show up as 76317 Mbytes in Fdisk and on your computer once you get it running. So if Fdisk reports your drive at 95.4% of the advertised size, don't let this concern you since this is normal and expected. If FDisk shows a number smaller than this, see the following 3 paragraphs for a workaround.

If you are working with a large disk up to 137GB, FDisk may not display the full size of the drive. I've found that FDisk will still work. You can use the following instructions in either case, when FDisk shows the full size and when it doesn't show the full size. If FDisk shows the full size (95.4% of advertised size) you can use either Mbytes or percentages in the following examples.

When working with large drives where FDisk does not show the full size of the hard drive you should use percentages for the partition sizes rather than the number of megabytes. If you are going to have only one partition, then after the size of the hard drive size is calculated and you are asked if you want to use the maximum available size for a Primary Dos partition and make the partition active (Y/N), answer N and press Enter. Another calculation will be made and FDisk will ask you to enter a partition size in Mbytes or percent of disk space (%) to create a Primary DOS Partition. Plug in 100% and press Enter and Esc. When you are back to the FDISK Options screen, choose option 2, press Enter, set partition 1 as active, press Enter and Esc.

If you had a 100GB hard drive and wanted to split it into 2 partitions with the first partition being 15GB and the second partition being 85GB, then after the size of the hard drive is calculated and you are asked if you want to use the maximum available size for a Primary Dos partition and make the partition active (Y/N), answer N and press Enter. Another calculation will be made and FDisk will ask you to enter a partition size in Mbytes or percent of disk space (%) to create a Primary DOS Partition. Plug in 15% and press Enter and Esc. To create the second Extended Dos Partition select option 1 and press enter. On the next screen select option 2 and press enter. After the size of the remaining unused hard drive is calculated, you are asked to enter a partition size in Mbytes or percent of disk space (%) to create a Extended DOS Partition. Plug in 85% and press Enter and Esc. Another calculation will be made for defining your logical drive(s). When prompted to enter a logical drive size in Mbytes or percent of disk space (%), plug in 100% and press Enter and Esc. When you are back to the FDISK Options screen, choose option 2, press Enter, set partition 1 as active, press Enter and Esc.

I have been able to make this work. When you format the 2 partitions, you should see the full numbers for each partition. Keep in mind the 95.4% rule. If this fails to work properly, you should use the Floppy or CD that came with the hard drive to partition the drive.

Since FDisk does not support any resizing activities you may also have to use the Delete DOS Partition or Logical DOS Drive option 3:

Operation on this menu is pretty straight-forward. You must first delete all logical DOS drives before deleting the Extended DOS Partition that they live in. This menu is also the first place we see any support for 'foreign' partitions. FDisk will recognize (some) non-DOS partitions if they exist on the drive. Although you can't create them, you can try to delete them. I don't believe FDisk is always completely successful when working with non-DOS partitions.

Option 4 will display the partition information for the currently selected disk drive. Shown is a primary drive with a non DOS partition, an active (A) bootable primary partition and an extended DOS partition:

and its logical drives:

A secondary drive with no primary partition:

and its logical drives:

Option 2, Set Active Partition is the last thing to do:

This is used to set which partition on the disk is considered active (remember, there can be only one active partition on a disk). Do not forget to set the active partition or your computer will not boot.

This is another place where non-DOS partitions will (may?) show up if they exist. In the case of this example the non-DOS partition is a hidden copy of the active boot partition. In theory you can use FDisk to mark it as active. I do not know what, if anything, it does to the partition type. It may reset it to be a standard active partition type but since it does not permit hiding the previously active partition I'm not sure how it would turn out.

Note: It is recommended that the system be rebooted after exiting FDisk. Failure to do this can result in incorrect information being displayed in FDisk and possibly inconsistent or corrupted partition information.

Well, there you have it, the basic partitioning operations using FDisk. Format the partition(s), install your operating system and enjoy.

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Last modified: June 19, 2014