there is a workaround in the RB bootloader which should stop this if its booted with the usb plug connected, but for some reason this isnt 100% reliable.
possible fixes:
dont put music in the MUSIC folder
make the SYSTEM folder read only
dont use file types the OF can read (i.e dont use mp3, wma)
edit: i've been told in IRC that the c200 rebuilds the DB before rebooting from MSC mode, unlike the e200 which rebuilds the DB when the OF loads.. so rockbox cant do anything about it.
making the SYSTEM folder readonly doesnt eliminate it, but speeds it up.