Question

I am trying to initialize and partition an attached virtual hard disk through the Windows API. I have been successful using DeviceIoControl() to do so, however whenever I apply the desired drive layout Windows is automatically assigning a drive letter to the partition and popping up an annoying “Would you like to format?” dialog.

My intent is to handle the formatting and mounting of this partition later in the program, but I’m not sure how to stop this behavior. I have tried setting RecognizedPartition to FALSE, but this seems to have no effect.

Relevant code:

Layout.PartitionStyle = PARTITION_STYLE_MBR;
Layout.PartitionCount = 4;
Layout.Mbr.Signature = MY_DISK_MBR_SIGNATURE;
Layout.PartitionEntry[0].PartitionStyle = PARTITION_STYLE_MBR;
Layout.PartitionEntry[0].PartitionNumber = 1;
Layout.PartitionEntry[0].StartingOffset.QuadPart = MY_DISK_OFFSET;
Layout.PartitionEntry[0].PartitionLength.QuadPart =
   (Geom.DiskSize.QuadPart - MY_DISK_OFFSET);
Layout.PartitionEntry[0].Mbr.PartitionType = PARTITION_IFS;
Layout.PartitionEntry[0].Mbr.BootIndicator = FALSE;
Layout.PartitionEntry[0].Mbr.RecognizedPartition = FALSE;
Layout.PartitionEntry[0].Mbr.HiddenSectors =
   (MY_DISK_OFFSET / Geom.Geometry.BytesPerSector);

for (int i = 0; i < 4; i++)
{
    Layout.PartitionEntry[i].RewritePartition = TRUE;
}

if (!DeviceIoControl(hDisk, IOCTL_DISK_SET_DRIVE_LAYOUT_EX,
                     Layout, dwLayoutSz, NULL, 0, &dwReturn, NULL))
{
    // Handle error
}
DeviceIoControl(hDisk, IOCTL_DISK_UPDATE_PROPERTIES,
                NULL, 0, NULL, 0, &dwReturn, NULL);

What can I do to prevent automatic drive letter assignment?

Answer 1

The only reliable way I could find to work around this issue was to stop the “Shell Hardware Detection” service while the volume was created and formatted. However, this approach is so unapologetically silly that I refused to put it into code.

Another “hackish” option is to have the service start up and then immediately spawn itself (or a “worker” executable) in a hidden window via CreateProcess() with the CREATE_NO_WINDOW flag.

Since this software runs as a system service and I’d rather not complicate the code for something that only happens once or twice over the lifetime of the system, I’ve just had to accept that sometimes there will occasionally be an Interactive Services Detection window pop up for a few moments while creating the partitions.

If anyone discovers a good method for preventing the format prompt while programmatically creating and formatting a drive, I’ll happily change the accepted answer (and owe you a beer).