And this:
"Does USB work?
USB currently isn't implemented for the Sansa. Rockbox can detect when the USB cable is connected but doesn't currently do anything with it. When you plug in a USB cable while the Sansa is off, the bootloader will detect this and boot the original firmware so that you can charge and upload files to your Sansa. Native Rockbox USB support is a current Google Summer of Code project and hopefully will be integrated in the not too distant future. If you plug in the USB cable while Rockbox is running, Windows will pop up a "Drivers not found" window which should be ignored."
From the Sansa FAQ.