You'll need a crossover cable to directly: PS3-1 > PS3-2 >> Data << transferring.
Or two regular UTP cable; but from a LAN port on your router/modem. Bridge, set up the IPs manually.
Make a Routing path in your router/modem config; (i.e 192.168.x. - 10.0.x.x)
But can't you just screw up; one screw if it's the slim, and none if it's FAT, replace HDDs, and copy files? Perhaps use one of the FTP, Remote softwares here. Just manually assign the IPs; and open up Port 21 on your router/modem.
All I can think of, else it's back to the basics with external storage components.
* Last resort, you can always take out the HDD you want stuff on, copy the folders, or at least make the same dir. tree for easier copying; connect your HDD to your computer with a SATA cabel, to any SATA port; and boot to Windows and have fun. (It's not fun I know). You can easily just let the PS3 HDD; hang/rest on something inside your cabinett. Plug the HDD with power from your PSU, usually the same cable attached to your DVD/CD/BD-ROM Burner (whatever), main HDD (C:\).
That's the hard part... Now I'm not so sure anymore, I mean, if neither of that worked for me, I would give up!