(From Marc Reinig) Look at the code that Devcon.exe (from the DDK) uses in 
Devcon.exe find =diskdrive 

This will give you the instance ID of the disks present on the system, in 
order of their disk number. 0, 1, 2, 3, … . So now you know what the disk 
number is for your device. 

Then, using the code below, loop through all possible 26 logical drives, 
using strings that look like “C:” for szDriveLetter. When you get an 
sdnDevNumber.DeviceNumber that matches the disk number you are looking 
for, you have found your drive letter, szDriveLetter: 

Code: 

char szDriveLetter [ 3 ]; 
char * pszDrive; 
STORAGE_DEVICE_NUMBER sdnDevNumber; 
BOOL fResult; 

fResult = GetBasicDiskDriveMapping ( szDriveLetter, &sdnDevNumber ); 

// sdnDevNumber.DeviceNumber will contain the drive number you want 
// sdnDevNumber.PartitionNumber will contain the partition for your reference 

BOOL GetBasicDiskDriveMapping ( LPCSTR pszDrive, STORAGE_DEVICE_NUMBER *psdn )

{ 

	BOOL fResult; 
	char szDriveName [ 7 ]; 
	HANDLE hDrive; 
	DWORD dwBytesReturned; 

	__try 
	{ 
		lstrcpy ( szDriveName, "\\\\.\\" ); 

		lstrcat ( szDriveName, pszDrive ); 

		// Open the volume to which the drive letter refers and get 
		// the physical drive on which the volume resides. 

		hDrive = CreateFile ( szDriveName, GENERIC_READ, 

		FILE_SHARE_READ|FILE_SHARE_WRITE, 0, 

		OPEN_EXISTING, 0, 0 ); 

		if ( INVALID_HANDLE_VALUE != hDrive ) 

			fResult = DeviceIoControl ( hDrive, 

			IOCTL_STORAGE_GET_DEVICE_NUMBER, 0, 0, 

			psdn, sizeof (STORAGE_DEVICE_NUMBER),

			&dwBytesReturned, 0 ); 

		else 

			fResult = false; 
	} 

	__except ( EXCEPTION_EXECUTE_HANDLER ) 

	{ 
		// if we get here, one of the arguments probably 
		// points to not enough memory. 

		SetLastError ( ERROR_INVALID_PARAMETER ); 

		fResult = false; } 

		CloseHandle ( hDrive ); 

		return ( fResult ); 
	}