Default Treating USB storage as internal? (Android Media Scanner)

Hi, I recently purchased one of these Android 4.1 Car stereo units (I've looked all over that thread and haven't been able to find a solution to my problem.)

I want to be able to use Doubletwist player as my main music player, and I want it to be able to read the files on my external usb hard drive (plugged into the back of the unit).

The unit recognizes the USB hard drive, as I can access all the files through certain other music players and through any file explorer. The problem is that Doubletwist won't recognize anything other than the internal storage and the SD card.

I emailed Doubletwist and they replied with this:
DoubleTwist Player uses two sources to recognize files (& the associated metadata) and playlists on the device.

1. Sync database created/updated by the computer application during a sync.
2. Android Media Scanner (AMS) database which is updated by the Android Media Database service.

So if the storage location isn't seen by the AMS service, doubleTwist Player will not import information about these files into it's database.
Is there any way to edit what directories the Android Media Service looks at? Then I could simply swap the sd card and usb drive directories (I don't use the SD card slot at all). Or does anyone have another solution to get this to work?

Alternatively, I just want decent music player similar to Doubletwist. I like Doubletwist's intuitive UI, the fact that it uses playlists and the fact that I can "play next" or "add to play queue".

I have tried Poweramp and I don't really like the interface, and Rocketplayer would have been fine but it is really buggy.

Any help would be greatly appreciated, thanks.

P.S. Forgot to mention, the usb directory is /mnt/usb/mnt1