If you can put the squashfs root, configuration YAML, and ISO on a web server, you should be able to use a grub command line similar to the PXE boot option with a couple HTTP URLs included in the kernel options. The kernel command line will tell the kernel where to load the squashfs root from, the config YAML URL will also be an option on the command line and then the configuration YAML will tell harvester-installer where to go to get the ISO. The only thing you should need on the hard drive would be grub, the kernel, and the initrd. If you don't have access to a webserver, things get more complicated. I have installed Harvester off a local SSD (including the ISO), but it required repackaging the initrd to include the squashfs root because I couldn't figure out how to get the squashfs root loaded off the hard disk with SLE Micro 5.3. If you actually want to install back to the same drive/SSD, then it gets a bit more complicated because you will need to copy the ISO off the drive to RAM before the installer runs, since the installer will take over the drive you just booted from.