Sorry, you really need the program HDtoolbox and this can only be found on the disc called `Install 3.1´ and not on ´Workbench 3.1´ ...
If you only can boot with your Workbench floppy then you may be able to do the following :
check in your directory Storage:Dosdrivers if there is a file called ´PC0´ and copy this to the directory Devs:Dosdrivers and then reboot. Now you have made your DF0 internal drive also PC compatible, meaning that you can insert a 720Mb PC formatted floppy, and before you do that you can search on Aminet (using your pc probably) for a HDtoolbox-like program and copy that on the floppy that you can insert in your Amiga. It´s a clumsy way, I know, but an Amiga does not have standard support for a bootable CDrom like Windows things do ...
Another thing you can do is to ask a friend who has got an Amiga with kickstart 3.1 and OS3.9 to make an ´Emergency Disk´ that allows you to boot and install your harddrive ...
