Vista and Windows 7 block write access to a mounted volume unless it's locked first: http://msdn.microsoft.com/en-us/library/cc835968.aspx

As that article states, you can either unmount it first or lock it during the write operation and unlock it when finished. This example uses the lock method:

Code:
Option Explicit

Private Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" (Destination As Any, Source As Any, ByVal Length As Long)
Private Declare Function SetFilePointer Lib "kernel32" (ByVal hFile As Long, ByVal lDistanceToMove As Long, lpDistanceToMoveHigh As Long, ByVal dwMoveMethod As Long) As Long
Private Declare Function ReadFile Lib "kernel32" (ByVal hFile As Long, lpBuffer As Any, ByVal nNumberOfBytesToRead As Long, lpNumberOfBytesRead As Long, ByVal lpOverlapped As Long) As Long
Private Declare Function WriteFile Lib "kernel32" (ByVal hFile As Long, lpBuffer As Any, ByVal nNumberOfBytesToWrite As Long, lpNumberOfBytesWritten As Long, ByVal lpOverlapped As Long) As Long
Private Declare Function CreateFile Lib "kernel32" Alias "CreateFileA" (ByVal lpFileName As String, ByVal dwDesiredAccess As Long, ByVal dwShareMode As Long, ByVal lpSecurityAttributes As Long, ByVal dwCreationDisposition As Long, ByVal dwFlagsAndAttributes As Long, ByVal hTemplateFile As Long) As Long
Private Declare Function FlushFileBuffers Lib "kernel32" (ByVal hFile As Long) As Long
Private Declare Function LockFile Lib "kernel32" (ByVal hFile As Long, ByVal dwFileOffsetLow As Long, ByVal dwFileOffsetHigh As Long, ByVal nNumberOfBytesToLockLow As Long, ByVal nNumberOfBytesToLockHigh As Long) As Long
Private Declare Function UnlockFile Lib "kernel32" (ByVal hFile As Long, ByVal dwFileOffsetLow As Long, ByVal dwFileOffsetHigh As Long, ByVal nNumberOfBytesToUnlockLow As Long, ByVal nNumberOfBytesToUnlockHigh As Long) As Long
Private Declare Function CloseHandle Lib "kernel32" (ByVal hObject As Long) As Long

Private Const GENERIC_READ = &H80000000
Private Const GENERIC_WRITE = &H40000000
Private Const FILE_SHARE_READ = &H1
Private Const FILE_SHARE_WRITE = &H2
Private Const OPEN_EXISTING = 3
Private Const INVALID_HANDLE_VALUE = -1&
Private Const BytesPerSector = 512
Private Const FILE_BEGIN = 0


Private Function DirectWriteDriveNT(ByVal sDrive As String, ByVal iStartSec As Long, ByVal iOffset As Long, ByVal sWrite As String) As Boolean
    Dim hDevice As Long
    Dim abBuff() As Byte
    Dim nSectors As Long
    Dim nRead As Long
    Dim ab() As Byte
    nSectors = Int((iOffset + Len(sWrite) - 1) / BytesPerSector) + 1
    hDevice = CreateFile(sDrive, GENERIC_READ Or GENERIC_WRITE, FILE_SHARE_READ Or FILE_SHARE_WRITE, 0&, OPEN_EXISTING, 0&, 0&)
    If hDevice = INVALID_HANDLE_VALUE Then Exit Function
    abBuff = DirectReadDriveNT(sDrive, iStartSec, 0, nSectors * BytesPerSector)
    ab = StrConv(sWrite, vbFromUnicode)
    CopyMemory abBuff(iOffset), ab(0), Len(sWrite)
    Call SetFilePointer(hDevice, iStartSec * BytesPerSector, 0, FILE_BEGIN)
    Call LockFile(hDevice, LoWord(iStartSec * BytesPerSector), HiWord(iStartSec * BytesPerSector), LoWord(nSectors * BytesPerSector), HiWord(nSectors * BytesPerSector))
    DirectWriteDriveNT = WriteFile(hDevice, abBuff(0), UBound(abBuff) + 1, nRead, 0&)
    Call FlushFileBuffers(hDevice)
    Call UnlockFile(hDevice, LoWord(iStartSec * BytesPerSector), HiWord(iStartSec * BytesPerSector), LoWord(nSectors * BytesPerSector), HiWord(nSectors * BytesPerSector))
    CloseHandle hDevice
End Function


Private Function DirectReadDriveNT(ByVal sDrive As String, ByVal iStartSec As Long, ByVal iOffset As Long, ByVal cBytes As Long) As Variant
    Dim hDevice As Long
    Dim nSectors As Long
    Dim nRead As Long
    Dim abResult() As Byte
    nSectors = Int((iOffset + cBytes - 1) / BytesPerSector) + 1
    hDevice = CreateFile(sDrive, GENERIC_READ Or GENERIC_WRITE, FILE_SHARE_READ Or FILE_SHARE_WRITE, 0&, OPEN_EXISTING, 0&, 0&)
    If hDevice = INVALID_HANDLE_VALUE Then Exit Function
    Call SetFilePointer(hDevice, iStartSec * BytesPerSector, 0, FILE_BEGIN)
    ReDim abResult(cBytes - 1)
    ReDim abBuff(nSectors * BytesPerSector - 1)
    Call ReadFile(hDevice, abBuff(0), UBound(abBuff) + 1, nRead, 0&)
    CloseHandle hDevice
    CopyMemory abResult(0), abBuff(iOffset), cBytes
    DirectReadDriveNT = abResult '(0)
End Function


Private Function LoWord(ByVal dw As Long) As Integer
    If dw And &H8000& Then
        LoWord = dw Or &HFFFF0000
    Else
        LoWord = dw And &HFFFF&
    End If
End Function


Private Function HiWord(ByVal dw As Long) As Integer
    HiWord = (dw And &HFFFF0000) \ 65536
End Function


Private Function hex2ascii(ByVal hextext As String) As String
    Dim y As Integer
    Dim num
    Dim Value
    For y = 1 To Len(hextext)
        num = Mid(hextext, y, 2)
        Value = Value & Chr(Val("&h" & num))
        y = y + 1
    Next y
    hex2ascii = Value
End Function
Call it like this:

Code:
Private Sub Command1_Click()
    ' Write hex "EB" to the first byte of the MBR on PhysicalDrive2
    DirectWriteDriveNT "\\?\PhysicalDrive2", 0, 0, hex2ascii("eb")
End Sub
OR:

Code:
Private Sub Command1_Click()
    ' Write hex "EB" to the first byte of the first partition boot sector of PhysicalDrive2:
    DirectWriteDriveNT "\\?\PhysicalDrive2", 63, 0, hex2ascii("eb")
End Sub
Assuming that H: is the first partition on PhysicalDrive2, this does the same thing as the previous example:

Code:
Private Sub Command1_Click()
    ' Write hex "EB" to the first byte of the partition boot sector of volume H:
    DirectWriteDriveNT "\\?\H:", 0, 0, hex2ascii("eb")
End Sub
Code:
Private Sub Command1_Click()
    ' Write hex "EB" to the last byte of the partition boot sector of volume H:
    DirectWriteDriveNT "\\?\H:", 0, 511, hex2ascii("eb")
End Sub
Code:
Private Sub Command1_Click()
    ' Write hex "EB" to the last byte of the MBR of PhysicalDrive2
    DirectWriteDriveNT "\\?\PhysicalDrive2", 0, 511, hex2ascii("eb")
End Sub
Change "PhysicalDrive2" and "H:" to match your drive layout.