Default [Q] chown with adb/root access: operation not permitted

On Note II rooted and fresh install of Cyanogenmod 10.3:

I want to do a symlink to save from cameras to externalsd. So I did:

1) CW recovery mode and adb shell. Command id reveals uid=0(root) gid=0(root) so I absolutely must have root access.
And then:
mv /sdcard/DCIM /sdcard/DCIM2
mkdir /external_sd/DCIM
ln -s /external_sd/DCIM /sdcard/DCIM
the /external_sd/DCIM folder has root:root ownership with full access (drwxrwxrwx)

chown media_rw:media_rw  /external_sd/DCIM
results in "Operation not permitted"

I've made sure /system is mounted and I've tried the above after doing su for root mode (which I'm already in!)