I got some more information on this, see below:
On the CDS50, USB drives can be browsed via the front panel only. Further to this, the CDS50 does not have a web client to browse lists. USB drives are not available in MusicLife.
Broadly speaking, browsing from USB devices has always been a ‘niche’ feature for our markets. It is far more common that customers would be expecting to use NAS drives running UPnP servers, or the streaming services provided within MusicLife.