How can I wait for a media file to become available before running an Update command?

Ben Tiedt said:

Is it possible that these APIs are not properly managing files (disposing of file streams, for example)?

It is possible. Please take a look and let me know if you spot anything suspicious:

      public string ExtractTitle(string url)
        {

            ICentralizedFile file = null;

            if (CentralizedFileStorage.IsCentralizedFileUrl(url))
                file = CentralizedFileStorage.GetCentralizedFileByUrl(url);


            string path = ConfigurationManager.ConnectionStrings["FileStorage"] + file.FileStoreKey + "\\" + file.Path.Replace(".", "\\") + "\\" + file.FileName;

            var adapter = new JpegMetadataAdapter(path);

            return adapter.Metadata.Title;
        }
   

        public string ExtractKeywords(string url)
        {

            ICentralizedFile file = null;

            if (CentralizedFileStorage.IsCentralizedFileUrl(url))
                file = CentralizedFileStorage.GetCentralizedFileByUrl(url);


            string path = ConfigurationManager.ConnectionStrings["FileStorage"] + file.FileStoreKey + "\\" + file.Path.Replace(".", "\\") + "\\" + file.FileName;

            var adapter = new JpegMetadataAdapter(path);

            return String.Join(", ", adapter.Metadata.Keywords);
        }
		
		public static byte[] DrawMultilineWatermark(string url)
    {

        ICentralizedFile file = null;

        if (CentralizedFileStorage.IsCentralizedFileUrl(url))
            file = CentralizedFileStorage.GetCentralizedFileByUrl(url);


        using (var bitmap = new Bitmap(file.OpenReadStream()))
        using (var gr = bitmap.GetAdvancedGraphics())
        using (var textPath = new Path())
        {
            var watermarkColor = new RgbColor(255, 255, 255, 145);

            var fontSize = UnitConverter.ConvertPixelsToUnits(bitmap.DpiY, bitmap.Height, Unit.Point) / 20.0f;

            var watermarkText = string.Format("<span style=\"font-size:{0}pt\">© {1} Stock Photo Site.</span>", (int)(fontSize / 2), DateTime.Now.Year);

            var text = new PlainText(watermarkText, gr.CreateFont("Arial", fontSize));
            text.Alignment = TextAlignment.Right;
            text.Leading = 1;

            textPath.DrawText(text);

            var rect = new RectangleF(0, 0, bitmap.Width, bitmap.Height);

            float offset = text.GetBlackBox().Width / 2;

            using (var tiledTextPath = TilePathText(textPath, rect, offset))
            {
                gr.FillPath(new SolidBrush(watermarkColor), tiledTextPath);
                /*
                for (float i = offset; i < bitmap.Width; i += offset / 2)
                {
                    gr.DrawEllipse(new Pen(watermarkColor, 3), -i, rect.Height - i, i * 2, i * 2);
                    gr.DrawEllipse(new Pen(watermarkColor, 3), rect.Width - i, rect.Height - i, i * 2, i * 2);
                }*/
            }

            System.IO.MemoryStream stream = new System.IO.MemoryStream();
            bitmap.Save(stream, new Aurigma.GraphicsMill.Codecs.BmpSettings());

            return stream.ToArray();
        }
    }

Debugging custom code and 3rd party components is out of scope for support here, but, I will note a few things to consider:

1. Disable the metadata calls and try the completion script with no calls and then try each call individually to see if it is an issue with a specific call.
2. Looking at your custom code, I would recommend using stream instead of ever trying to recreate a local file path -- that path generation logic is not supported and could change.