USB Loaders use WBFS file system and MPlayer uses windows based FAT32 (or is it NTFS) file system.
Unless your external drive has 2 partitions it will not pick them up.
And I never had luck with Mplayer with my USB HDD.
I have to use Geexbox to load media from the HDD.
I use Mplayer CE for playing videos over wifi from my PC shares.
and another Mplayer to play off SD.
I have about 4 different media players installed.