Composite Models: Can't append Tables with different storage modes

We have received several reports of issues similar to this.  The underlying issue is that the query is being mistakenly classified as DirectQuery rather than import.  We're making some improvements to this such that this does not happen, but they aren't available just yet.

In the mean time, you can work around this by creating a query that must be import, such as one based on a CSV or Excel file.  Once that query is loaded to the model, go back to the query editor, right click on the query, select Advanced Editor.  Replace the query text for the CSV/Excel query with the query that does the append.