USB (directly connected HDDs) is not the best way to use the Stream Magic 6 / Minx Xi / NP30 The USB connections are there for convenience to let you connect a memory stick or hard drive, etc with some tracks on it for ad hoc playback.
The intended functionality of both the Stream Magic 6 / Minx Xi / NP30 is as a network player using UPnP.
When browsing a USB stick, you are simply browsing the file/folder structure. So, if you haven't alphabetised/categorised your music it makes it much harder to navigate.
For example, if you had 1000 random tracks dumped in no particular order on a USB drive, there would be no way that you could navigate by artist/album/etc...
However, using UPnP, the Stream Magic 6 / Minx Xi / NP30 looks at the categorised metadata that is made available by the server.
This means that you can now browse the same random 1000 tracks by artist/album/genre/etc (depending on what the server supports).
Of course, using the metadata also gives you added bonuses such as album art if using the Stream Magic Remote app to control the player.
Via this method the Stream Magic 6 / Minx Xi / NP30 also supports ethernet or wireless , which gives fast access to your files.
For 24-bit content we recommend you use wired Ethernet as the high data rates do tend to stretch most wireless networks to their limits.
For more information on metadata, see the FAQ here