Time-tested advice, for what it's worth: carry two pumps, not one. Rely on the electric one, since it's fast and easy--when you've got a flat, usually just before dark, in driving rain, or on a busy highway where stopping means risking your life, the last thing you're going to want to do is hang around hand-pumping.

But carry a mountain bike pump as backup. Advice given above is good: a large barrel, a hose for connecting to the valve. I'd add that a cheap one is wasted money and will let you down, so buy quality. Ziptie it to your frame so that you won't be tempted to lend it out or remove it at inopportune times. Sooner or later your electric pump will die.

Note that sooner or later your hand pump will also die, particularly if left out in the weather year after year. Don't neglect this simple fact.

Little CO2 cartridges are fine as long as you carry enough of them. They're good for popping a bead back into place if you ever go tubeless. You'll be surprised at how many cartridges it takes to inflate a big back tire, and how much they cost. You'll then need to find new cartridges each time you get a flat. On a long trip you might get a lot of flats. You can do the math, but I'll say that it takes two to inflate my tubeless mountain bike tires, and my motorbike tires are a lot larger than that.

Hope that's helpful.

Mark