As you've got a bad back I feel obliged to mention the PSR3000 as a lightweight keyboard that sounds good, has lots of features and possesses siginficant external storage capacity via USB.

However I suspect that the compromises in construction that give you a relatively lightweight keyboard will put you off. The 3k feels flimsy next to the Korg units and the keyboard action is somewhat spongy and takes some getting used to.

Personally I've been gigging a Yamaha 630, then 2k, now 3k for five years now and they all have basically the same build and keyboard action, without any reliability issues, and I can also live with the keyboard action.

If you think the Yamaha is worth a look, you MUST try it yourself to see whether you can live with the compromises.

Also the key action on Tyros1 and Tyros2 are much better; having recently played both I feel the T1 is rather light but the T2 felt spot on.