Ok..Need to see the device mounted by Kernel, if external, it can to be sdb, sdc, mmc, etc..Open up a terminal and type: fdisk -l and see output..Check the /de/sdX where X can be a,b,c,etc) and mmcblkX (X=0,1,2,3,etc) if any MicroSD memory.
USB devices mainly are /dev/sdbX Where X=1,2,3,etc. Also a second internal HDD will take /dev/sdb. In this case, HDD take priority.
Flash memory (MicroSD) are: /dev/mmcblkX X=0,1,2,etc.
To use an external HDD, need to see the wiki or google..Installing on extenal device has a different way of doing it...You will need to do it manually and then install grub2 or use some third party app...I would highly suggest to read before doing something.
In my case, I always avoid third party app, always do it manually or cli...