I just tried using the hd-media build to boot from an existing hard disk
and install without using removable media and was not able to do so
because I used btrfs on the hard disk, and most filesystems are packaged
as separate udebs rather than being built into the initrd.  I think it
was only iso9660, fat, and ext4 that were in there.  Please consider
moving btrfs and probably a few other filesystem modules into the
hd-media initrd so that it can be used outside of ext4 and fat.