Slightly confused, which app are you using that you want to put the songs on a USB?
If it is the desktop client, you can change the cache location where it stores its files in Edit > Preferences > Cache. Just be aware if you change that to a USB drive, that drive must always be present when you start Spotify otherwise it will create a new cache which makes the one on the USB drive completely useless.
OK that kinda helps. As I said, I want to listen OFFLINE on my iPad on long airplane trips and dont want to load up 3-5GB of music on my iPad. I have an iStick (http://www.hypershop.com/HyperDrive/iStick/) and I want to load it up (from Spotify via Offline mode) so that I can listen offline to Spotify.
I don't think you can move playlists on iPad onto a USB stick, although I've never really tried myself. I don't think you will be able to access the Spotify files or tell it to save in a specific place.
But the status is CASE CLOSED with the following comment:
"This is simply not possible because on a mobile device the Spotify app has to be installed to listen to Spotify tracks. The reason is: Artists need to get paid for every played song - that's only possible if the song is played "inside" Spotify."
Is there a reason why it wouldn't work on an Apple device? I would have thought if the iPad can take an external usb storage, then theoretically it should work. Unless of course Apple have ensured that the app behaves in different way to the Android one. Apologies if that's the case, I don't own any Apple products to test this out. Thanks for letting me know.