UI thread still hangs while using BackgroundWorker

Question

As a consequence of the problem described here:

WPD MTP stream hangs on commit

the UI of my WPF program is hanging (meaning it goes unresponsive while a certain task is performed), despite making the call using a BackgroundWorker. I have read something about this having something to do with the fact that the COM object on which the operations are performed is created on the UI thread, but when I break on the piece of code that hangs I see that it in fact is executed in a WorkerThread and not on MainThread, which seems to suggest that is not the problem (although I am a threading rookie).

Can anyone explain why this happens and how I should solve this UI hang?

Relevant code:

From base class, where the backgroundworker is set up:

public void Sync()
{
    SyncBackgroundWorker.WorkerReportsProgress = true;
    SyncBackgroundWorker.WorkerSupportsCancellation = true;
    SyncBackgroundWorker.DoWork += SyncBackgroundWorker_DoWork;
    SyncBackgroundWorker.ProgressChanged += SyncBackgroundWorker_ProgressChanged;
    SyncBackgroundWorker.RunWorkerCompleted += SyncBackgroundWorker_RunWorkerCompleted;

    SyncBackgroundWorker.RunWorkerAsync();
}

protected void SyncBackgroundWorker_DoWork(object sender, DoWorkEventArgs e)
{
    // Retrieve needed files from iTunes and compose m3u playlists
    var syncInfo = this.getSyncInfo();

    // Save m3u files to music root on target
    this.saveM3UPlaylists(syncInfo.M3Us);
}

From class that extends base class (the last line initiates the file copy):

protected override void saveM3UPlaylists(List<FileInfo> M3UPlaylists)
{
    foreach (var file in this.Folder.GetFiles())
    {
        if (file.Extension == ".m3u")
            file.Delete();
    }

    int numPlaylists = this.Playlists.Count;
    for (int i = 0; i < numPlaylists; i++)
        PortableDevices.PortableDeviceFile.CopyFileToDevice(this.Folder, makeValidFileName(this.Playlists[i].Name + ".m3u"), M3UPlaylists[i]);
}

From relevant portable device class. The targetStream.Commit(0) part causes the actual freeze, as described in the previously given link. Note that the the COM-object (IPortableDevice) parent.Device.Content has been previously initialized, when the connection with the device was set-up, at the start of the program.

public static void CopyFileToDevice(PortableDeviceFolder parent, string name, FileInfo file)
{
    IPortableDeviceValues values = GetRequiredPropertiesForContentType(parent.Id, name, file.Length);

    PortableDeviceApiLib.IStream tempStream;
    uint blockSize = 0;
    parent.Device.Content.CreateObjectWithPropertiesAndData(
        values,
        out tempStream,
        ref blockSize,
        null);

    System.Runtime.InteropServices.ComTypes.IStream targetStream =
        (System.Runtime.InteropServices.ComTypes.IStream)tempStream;
    try
    {
        using (var sourceStream = file.OpenRead())
        {
            var buffer = new byte[blockSize];
            int bytesRead;
            do
            {
                bytesRead = sourceStream.Read(buffer, 0, (int)blockSize);
                targetStream.Write(buffer, bytesRead, IntPtr.Zero);
            } while (bytesRead > 0);
        }

        targetStream.Commit(0);
    }
    finally
    {
        Marshal.ReleaseComObject(tempStream);
    }
    parent.Refresh();
}