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Full Discussion: Hard Drives and MBR
Special Forums Hardware Hard Drives and MBR Post 302816363 by Corona688 on Monday 3rd of June 2013 05:27:33 PM
Old 06-03-2013
I'll explain this first: A hard drive doesn't know or care what its contents are. If it gives you sector 123523 when you ask for 123523, it has fulfilled its responsibility.

You don't "install" an MBR, it's just there, in sector zero, because that's where your computer expects it to be by tradition. If sector zero doesn't have valid contents, your computer just won't boot from it. More modern things like GPT -- which can have partitions much larger than a basic DOS-style MBR would support -- have extra data elsewhere as well, at the end of the disk I believe...

You don't get to put partitions in the first 63 sectors just because that's how DOS-style partition tables work, they can't represent numbers any lower than that.

Some disks will come formatted with a sensible MBR. Some just fill it with zeroes. It doesn't matter much, since there's no barrier to doing what you please with it.

The only thing actually stopping you from writing over the MBR is the operating system. When you partition a device in Linux, it breaks it out into separate pretend-devices like /dev/sda1, /dev/sda2, and so forth. Theyr'e safely compartmentalized, they won't let you write beyond their bounds, but it's the OS doing that for your convenience, not the disk itself. You can use /dev/sda itself if you want to talk to the raw device, but almost nothing except a partition editor would need to do so.
 

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CFDISK(8)							 GNU fdisk Manual							 CFDISK(8)

NAME
GNU fdisk, lfdisk, gfdisk - manipulate partition tables on a hard drive SYNOPSIS
fdisk [options] [device] DESCRIPTION
fdisk is a disk partition manipulation program, which allows you to create, destroy, resize, move and copy partitions on a hard drive using a menu-driven interface. It is useful for organising the disk space on a new drive, reorganising an old drive, creating space for new oper- ating systems, and copying data to new hard disks. For a list of the supported partition types, see the --list-partition-types option below. It comes in two variants, gfdisk and lfdisk. Lfdisk aims to resemble Linux fdisk 2.12, while gfdisk supports more advanced disk operations, like resizing the filesystem, moving and copying partitions. When starting fdisk, the default is to run gfdisk. OPTIONS
-h, --help displays a help message. -v, --version displays the program's version. -L, --linux-fdisk turns on Linux fdisk compatibility mode. This is the same as running lfdisk. -G, --gnu-fdisk turns off Linux fdisk compatibility mode. -i, --interactive where necessary, prompts for user intervention. -p, --script never prompts for user intervention. -l, --list lists the partition table on the specified device and exits. If there is no device specified, lists the partition tables on all detected devices. -r, --raw-list displays a hex dump of the partition table of the disk, similar to the way Linux fdisk displays the raw data in the partition table. -u, --sector-units use sectors, instead of cylinders for a default unit. -s, --size=DEVICE prints the size of the partition on DEVICE is printed on the standard output. -t, --list-partition-types displays a list of supported partition types and features. The following options are available only to lfdisk. -b, --sector-size=SIZE Specify the sector size of the disk. Valid values are 512, 1024 and 2048. Should be used only on older kernels, which don't guess the correct sector size. -C, --cylinders=CYLINDERS Specify the number of cylinders of the disk. Currently does nothing, it is left for Linux fdisk compatibility. -H, --heads=HEADS Specify the number of heads of the disk. Reasonable values are 255 or 16. -S, --sectors=SECTORS Specify the number of sectors per track. A reasonable value is 63. BUGS
Before editing a BSD disklabel, the partition with the disklabel should already exist on the disk and be detected by the OS. If you have created a BSD-type partition, you need to write the changes to the disk. If fdisk fails to notify the OS about the changes in partition ta- ble, you need to restart your computer. As fdisk tries to guess the device holding the BSD disklabel, it might fail to edit it at all, even if the OS has detected it. In this case you are adviced to simply open the device with fdisk directly. It is possible that it doesn't work on some operating systems. Getting the size of a partition with -s might fail, if fdisk fails to guess the disk device, for the same reasons as with the previous bug. SEE ALSO
mkfs(8), cfdisk(8), parted(8) The fdisk program is fully documented in the info(1) format GNU fdisk User Manual manual. fdisk 18 August, 2006 CFDISK(8)
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