CreateFile: direct write operation to raw disk "Access is denied" - Vista, Win7

While the answer of @MSalters makes sense, it is not how my code works. In fact it is so counter-intuitive, I spent several days making sure the code does in fact work.

These code snippets are in a proven, mass consumer market software product. When it needs to modify an on-disk structure, it dismounts the win32 volume so it can modify NTFS or FAT filesystem structures. Interestingly, the volume access handle is read-only:

    char    fn [30];    snprintf (fn, sizeof fn, "\\\\.\\%s:", vol -> GetVolName ());    vol_handle = CreateFile (fn, GENERIC_READ,                            FILE_SHARE_READ | FILE_SHARE_WRITE, NULL,                            OPEN_EXISTING,                            FILE_FLAG_NO_BUFFERING | FILE_FLAG_RANDOM_ACCESS,                            NULL);    if (vol_handle == INVALID_HANDLE_VALUE)    {          // show error message and exit    }

If unable to get write access to a volume or partition, this code forces a dismount if the user authorizes such after a stern warning:

if (!DeviceIoControl (vol_handle, FSCTL_DISMOUNT_VOLUME,                                            NULL, 0, NULL, 0, &status, NULL)){    DWORD err = GetLastError ();    errormsg ("Error %d attempting to dismount volume: %s",                                                        err, w32errtxt (err));}// lock volumeif (!DeviceIoControl (vol_handle, FSCTL_LOCK_VOLUME,                                            NULL, 0, NULL, 0, &status, NULL)){     // error handling; not sure if retrying is useful}

Writing is then fairly straightforward, except for positioning the file pointer by 512-byte sector:

    long    hipart = sect >> (32-9);    long    lopart = sect << 9;    long    err;    SetLastError (0);       // needed before SetFilePointer post err detection    lopart = SetFilePointer (vol_handle, lopart, &hipart, FILE_BEGIN);    if (lopart == -1  &&  NO_ERROR != (err = GetLastError ()))    {            errormsg ("HWWrite: error %d seeking drive %x sector %ld:  %s",                            err, drive, sect, w32errtxt (err));            return false;    }    DWORD   n;    if (!WriteFile (vol_handle, buf, num_sects*512, &n, NULL))    {            err = GetLastError ();            errormsg ("WriteFile: error %d writing drive %x sectors %lu..%lu:  %s",                            err, drv, sect, sect + num_sects - 1,                            w32errtxt (err));            return false;    }

It's quite rare to want only GENERIC_WRITE. You most likely want GENERIC_READ|GENERIC_WRITE.

There is note in MSDN in documentation of CreateFile:

Direct access to the disk or to a volume is restricted. For more information, see "Changes to the file system and to the storage stack to restrict direct disk access and direct volume access in Windows Vista and in Windows Server 2008" in the Help and Support Knowledge Base at http://support.microsoft.com/kb/942448.

It refers to Vista/2008, but maybe apply to Win7 also.