Internal hardrives in PC

I am using a desktop PC to run Volumio.
The internal HD is not big enough to host all my musicfiles.
Currently I am using an external HD to host the files.

If I install a second HD (large enough) can this one be added to INTERNAL folder.

Do I have to performe some editing of some config file to get this working?

I have not tried an x86 installation of Volumio, so I don’t have a picture of what your existing setup looks like (in terms of storage), but it should be quite straightforward to mount an additional hard drive (in fstab for example) and make a symbolic link from the drive so that Volumio/mpd finds it ok.