I emailed Blusesound as to whether a USB hard drive canbe connected to a Node2 and they rresponded:
External hard drives/USB Stick
work with the player's USB port. If there's music files in the
external HDD or USB Stick, we'll automatically index the songs and you can play
them directly on your player. Also, they only create a temporary index which
acknowledges the files on your USB drive separate from your library of music of
computer shared folders and NAS devices. As long as the hard drive is in either
FAT32(FAT) or NTFS format, our player will be able to read them.
For a full illustrations of what file formats Bluesound supports, please see
the following - https://support.bluesound.com/hc/en-us/articles/200347887-What-Formats-are-Supported-by-Bluesound-
So the good news is that, formatted correctly, an external USB drive should work.
Thanks for everyone's responses.