There is no external hard drive mounted on the system. Everything in /media is being stored on your main boot and OS partition, same as everything else in / (except /home/, which is on sda7). You should mount this external drive you want to be using for media, and move everything in /media onto it.
As for your above output of df, the discrepancy between total, used, and available space is probably due to root's reserved sectors, which by default is 5% of the drive (9.4GB in your case). The "available" space will always give you the result of total minus used minus reserved. Total is 188, used is 180, and reserved is 9.4, so there is zero space "available" for general use. You can reduce the amount of reserved space with tune2fs, but you really should be moving that media directory off of the main OS partition.