Code in C to Access the master boot record (MBR)

The format of the MBR can be found at Wikipedia[^]

In code these look like

//These structures must be packed with 0 bytes padding.
//I think the keyword "packed" does this on the turbo compiler
typedef packed struct _Partition {
	BYTE status; //0x80 for bootable, 0x00 for not bootable, anything else for invalid
	BYTE StartAddrHead; //head address of start of partition
	WORD StartAddrCylSec; //(AddrCylSec & 0x3F) for sector,  (AddrCylSec & 0x3FF) for cylendar
	BYTE PartType;
	BYTE EndAddrHead; //head address of start of partition
	WORD EndAddrCylSec; //(AddrCylSec & 0x3F) for sector,  (AddrCylSec & 0x3FF) for cylendar
	DWORD StartLBA; //linear address of first sector in partition. Multiply by sector size (usually 512) for real offset
	DWORD EndLBA;   //linear address of last sector in partition. Multiply by sector size (usually 512) for real offset
} Partition;

typedef packed struct _MBR {
	BYTE Code[440];
	DWORD DiskSig; //This is optional
	WORD Reserved; //Usually 0x0000
	Partition PartTable[4];
	BYTE BootSignature[2]; //0x55 0xAA for bootable
} MBR;

Both of these structures must be packed. That is there is no padding added by the compiler for optimisations. I think the keyword packed before struct does this on the Turbo compiler.

So, just open the drive and read/write.

To open the whole harddrive for writing use the following filename:
Windows: \\.\PhysicalDriveX where X is the drive number, starting from 0 (rember that to get a \ in code, you need 2 \, so this is \\\\.\\PhysicalDriveX in the source code).
Linux/UNIX/OSX: /dev/sdX where X is the drive letter, starting from a.