Using SD as an external storage

Hey I am trying to use the method discussed here:

I am following the instructions, I have named my SD card ‘BBB_Ext’ and my uEnv.txt looks as follows:

mmcroot=/dev/BBB_Ext ro

I have also put the following line in the fstab file:

/dev/mmcblk0p1    /media/card     auto     auto,rw,async,user,nofail  0  0

It remains on with 3 LED and won't boot. It works fine without the SD card in (I can SSH into it no problem).