In stock ROMs we know that external sdcard is mounted as sdcard/external_sd . When you root your device and flash CyanogenMOD you’ll find that external card is mounted as /mnt/emmc and internal sdcard as /mnt/sdcard.   Don’t worry it is a little bug in CyanogenMOD. When I rooted my phone I was very confused to see this change.

You can easily fix this bug. Henceforth you can either treat external sd card as internal storage and internal sdcard as external storage, which is best way to fix this bug!!! Or you can very easily mount /mnt/emmc to /mnt/sdcard/external_sd  as in stock ROMs

How to mount /mnt/emmc as /mnt/sdcard/external_sd:

1.Install terminal emulator by clicking here from google play  or scan the following QR code:

I guess, most of you may already have a terminal emulator installed. So skip this step.

2. Now open terminal emulator and excute following command:

                           su

mount -o bind /mnt/emmc /sdcard/external_sd

You need to create a directory named external_sd in the sdcard before executing this command. Create new directory using either file explorer app or  mkdir command in the terminal emulator.

Thanks to Tanx  for asking , You can undo this action by executing following command:

umount /mnt/sdcard/external_sd

EDIT

Many users are having trouble mounting their sdcard. Diffrent version of  CyanogenMOD has different path to SDcard. Thanks to Vincent Watertramp for his comment  on my post How to execute a command automatically at boot [Android]  and mentioning the external SD path for different CyanogenMOD  version:

CyanogenMod 7 : /mnt/emmc
CyanogenMod 9 : /mnt/emmc
CyanogenMod 10 : /mnt/extSdCard
CyanogenMod 10.1 : /mnt/extSdCard

p.s: I am on CyanogenMOD 10 on Samsung Galaxy SL i9003, this file explorer is ES file explorer.