Actually, when you are using Windows Azure AppendBlob and Windows Azure Data Factory, you are probably facing this issue: 

Error in Activity: Unknown error in module: System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.TypeLoadException: Could not load type 'Microsoft.WindowsAzure.Storage.Blob.CloudAppendBlob' from assembly 'Microsoft.WindowsAzure.Storage, Version=4.3.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'

Let's see what we can do about this.