Atari800 and saving to disk

I'm completely puzzled with the Atari800 emulator.

I've written a Basic program and can successfully save it to cassette by using the CSAVE command, and CLOAD for loading respectively. So far so good. 

However, I wanted to try the disk emulation as well. So I created a blank .ATR disk from the disk management menu, and inserted it in the D1: slot. I tried saving it using SAVE "D:TEST.BAS", SAVE "D1:TEST.BAS" but to no avail.

Am I missing a step here? I've configured the Atari as an Atari800xl 64kb machine with Basic loading as default.

Any help is much appreciated!