Cubase requires a computer or laptop to run.
Cubasis is the version that runs on an iPad.

I'm not clear on what you are asking. But the Montage can stream stereo audio files direct from a USB stick, it can write audio to one in realtime, as well.
MIDI files can be loaded and played by the internal Song PLAY/REC function.

I write a sequence I have a song,I want to record the song on the USB stick,then take the USB to a computer and play/listen to what I've recorded from the song

I'm assuming you want to play a .WAV file you created or otherwise acquired from some other source and want to put that .WAV file on a USB stick - plug the USB stick into Montage - and have Montage play that .WAV file.

This is what I did:

1) Downloaded a public domain MP3 file (was looking for a WAV - but found MP3) Source: http://www.foldedspace.org/weblog/2006/06/in_the_good_old_summertime.html

Recording date was stated as 1905 - before copyright law took effect for recordings.

2) Converted the MP3 to a WAV using Audacity. Did this by loading the MP3 then exporting audio as a 16-bit PCM WAV file.

3) Saved the resulting WAV file to a random (old) USB stick on the (drive=X) X: (root) directory.

4) Unplugged from PC and plugged USB stick into Montage.

5) Pressed the sideways triangle "Play" button which is under superknob. A shortcut to go to the recorder.

6) Used touchscreen to select "Audio", then touchscreen pressed "Audio Name" section to bring up the "Load" option - and touchscreen pressed the "Load" option

7) The WAV file I saved on the USB stick was seen (since it's in root of the USB stick) - so I pressed that file on the touchscreen

8) With the WAV file loaded, I could press the play button again to start the recording playing

9) Audio was too loud, so I used the DATA DIAL to turn down the volume on this menu - a parameter called "Audio Volume"

Pressing "Stop" is the same as "Pause" - since it does not reset the position. It just stops playing. Rewind/Fast Forward/Begin/End and other buttons do what you would expect.

From the Audacity "About" screen (minus credits):

Audacity 2.1.2
free, open source, cross-platform software for recording and editing sounds
http://audacityteam.org/
Audacity is a free program written by a worldwide team of volunteer developers. Audacity is available for Windows, Mac, and GNU/Linux (and other Unix-like systems).
If you find a bug or have a suggestion for us, please write to our feedback address. For help, view the tips and tricks on our wiki or visit our forum.

...

Audacity® software is copyright© 1999-2015 Audacity Team.
The name Audacity® is a registered trademark of Dominic Mazzoni.

There's a newer version that fixes memory leaks and other things that would be worth an upgrade - but I can't remember the last time I used the software - so old version it is for now.

Page 41 owners manual,recording performance as audio,steps 1-6 are setup step 7 is play the keyboard.i want to record a song I've previously written on the montage to the USB without playing.

So you want to play the keyboard - have the keyboard record while you play (I'd suggest a MIDI file since it's smaller) and then have the keyboard play it back? Then use the recorder to record your playing (press record under the MIDI menu) press the song name to rename the MIDI file (it will be saved to "NewSong##.mid" if you don't). Then load the MIDI file in a similar way as the audio file - but use the MIDI tab.

"Previously written" is confusing as all get out.

You either have a file you've previously saved (WAV, MIDI, ?? something not compatible) or you need to generate this file from within the recorder (either a WAV or MIDI). Then you can play it back with the same recorder as outlined.