I believe your talking about something like the GameHunter on this page
http://www.videogamecompany.com/acat..._Parts_65.html (There are many different ones, that was just the first I saw). Works fine for me, tho you need an old model PlayStation (not the small PSOne in other words), and it has to have an I/O port on the back (which means no 9000 series). The only flaw is you gotta insert an original disc (any disc, even a demo) forst, wait for it to stop (bout 5 seconds), then insert your backup... nice if you don't want to open up your PlayStation.
You can also get a boot disc like the PS-X Change which does the same thing but on a disc.