I just press MEDIA on the remote and the on-screen display shows HDD plus a bar and %age figure for how full the disk is (for the Video/Radio allocation which is the normal default).
Pressing the coloured buttons <yellow or blue> for the Music/photo shows the same for the disk space allocation that you preset during initial setup.
For me its currently 60% and 3% on my 1TB disk where I allocated a fair bit to backup some photos on the HDR and then never bothered to use it.
The custom firmware is not required for this... but it has many other uses.