This was originally posted in the DUO2 Backup Images section in the 'OpenATV 5 XBMC edition thread' but I noted you stated having a Solo2 which is why I moved it here as there is no way you could have loaded 'this nice image' to your device.
Anyway, I have some movies on a portable USB HDD 320GB. It appears as 'External' on my Duo2 and I can successfully play movies from it without issue.
So I would expect it to work on a Solo2 as well but without knowing what you were running before and are actually running now any further suggestions would be pure guess work.