I've made a few homebrew cables, no need to take the iriver apart. It's just computer USB -> iRiver charger jack. I leave my AC charger at home, use this cable to charge at work, in car, etc.
- Find a generic USB cable for computer, cut it in half. Keep the computer end, toss the camera/printer end.
- Find a cable with the right tip to fit into the iRiver. Cut that in half, save the tip, toss the rest.
In the USB cable there are 4 wires. The outer two are 5 volts, generally red/black with red=+ and black =- The other 2 are data lines, just forget about them. Carefully connect the 2 cables, solder it together so it's 5V to the tip with + on the inside, - on the outside. Double and triple check the polarity before you plug it into your iRiver.