You cannot charge the Korg MR-1 via USB.
You are absolutely correct. However, that wasn't the purpose of my post! From the thread on using an external battery with the MR-1, the unit requires power that turns out to be compatible with USB power (5V, 800-500mA). The main problem is that you need some way to connect the USB source to the DC-in jack.
While it is possible to hack together a USB cable and a plug, the cable that I linked is a low-cost solution that doesn't require any cutting and soldering. In addition, it is a Y-cable, with the other end terminating in a mini-usb - this carries the data.
So the main benefit of this cable is that you can plug one end into your computer, and then the other two plugs into your MR-1. Now you can mount the drive and charge the unit -- no AC adapter required (which is almost the size of the MR-1).
The second benefit is that you can plug the MR-1 to an external Li-Ion battery pack. I just happened to have one, and I find it more convenient than the Korg external battery pack. I will still pack along both.