Right after you press the power button to turn it on, hold the << (rewind) button. A bunch of text should come up and in a few seconds the original firmware will load.
Go to settings > USB mode > select MTP
My computer > Sansa c200 > Whatever drive is there right-click it > Format
Selecting format will get rid of everything on your Sandisk (including rockbox), unplug your Sansa, replug it in. The MUSIC, VIDEO, PHOTOS, etc folders will be back but empty.
It's best to back up your files, format, then throw them back on your player with the original firmware.