well if that MSC mode is not available in your original sansa firmware the first thing i would probably do is downgrade to a different version of the original sansa firmware and then reinstall rockbox.
they have different original firmware files for your c200 series here... http://daniel.haxx.se/sansa/c200/firmware.mi4 (that's v1.00.06)
they have more here if you need them... http://files.zefie.com/PMP/sansa/c200/v1/firmware/ (but the one above should be fine) (also depending on where you live the A = America ... E = Europe) (if you aint sure what one you need you can check your current firmware version and find out if it's A or E )
because im guessing your current original sansa c200 series firmware happens to be one of those versions that dont allow you to select the MSC mode etc that's needed. (i dont know details but i have heard certain original sansa firmwares dont have those options)
you can probably use your devices 'recovery mode' to flash to a different version... just be careful!
here is the link to recovery mode... http://www.rockbox.org/twiki/bin/view/Main/SansaC200Port
Recovery mode can be used to replace a .mi4 image on the player. If you by some reason can't boot into the original firmware, like if you messed up the installation or similar, or you simply want to change the firmware to a different version, recovery mode can be your friend. To get there:
* Power off
* Turn ON lock/hold
* Hold REC button and press MENU button (used to turn the player on/off)
* Plug the player to the computer
* In My Computer you can see new drive 16MB-FORMAT
To update the firmware, copy the unzipped files into the 16mb drive, unplug the player, and release the hold switch.
[B]A word of caution WARNING
* Only copy these specific files to the recovery partition.
* Do not format this 16MB drive
* Do not try anything else unless you have to. Again: You might brick your player! [/B]
If all else fails, you can try the SansaC200Erase tool to completely wipe your Sansa clean in order to recover it.
so basically when in 'recovery mode' copy the firmware.mi4 file (http://files.zefie.com/PMP/sansa/c200/v1/firmware/1.01.06/SansaC200A1_01_06.7z (it's in .7z format so you will need www.7-zip.org to extract the .mi4 out of it) , if your in USA ... check your current firmware version as it should most likely show either A or E in it etc) to the device when it's in recovery mode and it will flash the player with a original firmware file and then your device will be back to stock. you will have to reinstall rockbox's bootloader (which you need to change your player to MSC Mode FIRST!) etc etc to get rockbox up and running again.
but the basic point where trying to do is get MSC mode back onto your c200 player because without MSC mode you cant really do to much with your c200 as far as Rockbox and file transfers etc etc.