here's my mini-guide for playing PGEN from the hdd, and of course roms from the HDD too.
step 1) install pgen with winhiip, i've attached an .nrg image file. i dont remember if winhiip will let you install .nrg's, if not mount with daemon and install from cd-rom. (id = PGEN_12.ELF) by the way, this is the newest pgen, i got it from codejunkies, i just put the elf in to an old disc image of pgen.
step 2) use dms hdd tool and create a partition on your hdd called "PGEN", i made mine 128mb, you can make yours whatever size you need to fit all your roms.
step 3) fire up execftps, connect to your ps2 with flashfxp, once connected press "CTRL + R" it will come up with a dialogue box, put this in there "SITE MNT /pfs/0/ hdd:+PGEN" (no quotes), it should say it successfully mounted. go in to "pfs", then go in to "0", transfer all of your roms.
step 4) fire up hdloader, run pgen, check out your rom list... in addition to cd-rom, memory card 1, memory card 2, pendrive, there is the very very sexy "PGEN".. highlight her & press X.. there's all your roms
step 5) play genesis til you vomit.