This is my code that I am using:

var fileList = this.SliderPages;
                string serializedString = string.Empty;

                if (fileList.Count > 0)
                {
                    using (MemoryStream tempMemory = new MemoryStream())
                    {
                        BinaryFormatter binaryFormatter = new BinaryFormatter();
                        binaryFormatter.Serialize(tempMemory, fileList);
                        serializedString = System.Convert.ToBase64String(tempMemory.ToArray());
                    }
                }

                DataFactory factory = EPiServer.DataFactory.Instance;
                var data = factory.GetPage(this.CurrentPage.PageLink);

                data.SetValue(this.PropertyName, serializedString);

                factory.Save(data, EPiServer.DataAccess.SaveAction.Save);

This is the error I get:

Message = "The property SliderBox is read-only."

Before you can set new value. you need a writeable clone of the page data object:

var writablePage = data.CreateWritableClone();