I don't know about using the CCA to access files from a HD - I personally would not bother. At the price the convenience of casting Spotify (but in my case CD quality Qobuz which can also be 'cast') is worth the beer money investment alone - and it's very reliable too. You can also cast radio stations in the same way by using the Tunein radio app.
I would have used the CCA to compliment the R-pi as mentioned earlier. In my system I use the same DAC for both a Squeezebox Touch and CCA; the CCA plugs into the optical input of the DAC while the SBT plugs into the coax of the DAC - they run side by side nicely. The SBT is king at streaming files from a Computer whereas the CCA is a more up-to-date source for streaming services such as Spotify and Qobuz.
Oh and if you're concerned about the low cost power supply with with the CCA - you can easily run battery powered from a portable mobile phone charger - it lasts hours and hours.