Prueba a ponerlo así... cambiando los dos...
# internal sdcard
dev_mount sdcard /storage/sdcard 28 /devices/platform/msm_sdcc.2/mmc_host/mmc1
# externel sdcard
dev_mount external_sd /storage/sdcard/external_sd auto /devices/platform/msm_sdcc.4/mmc_host/mmc2
