Hey that's cool! I have an FZ20M and was thinking of doing the same thing.... How big in MB is the partition? Will the FZ see a 500MB one? I think the original Mac HDs were no more than 80MB at the time.
I'm not sure it is possible to copy the files from a PC to the card in any way the FZ can read. You could dump by midi (slow), or use a special built cable - I have not done this. Or use the normal FZ floppy to load sounds and then save them to the HD. Probably best way I can think of is to have an HxC drive for the floppy, load sounds into the FZ from that, and save them to the HD. I have an HxC installed in my FZ1 and I am able to move .fzf files from the PC onto the SD card and then put the card in the FZ1.