Ah, gotcha. Back in the ADN days one fellow kept a 2008 iMac with a buggered internal HD operational for several years with an external Firewire 800 SSD.
I use a not-dissimilar theory with a Pismo G3 PowerBook that was given to me. An internal fault means it can't recognise and HDs in it. So I use an external Firewire 400 CompactFlash card reader with a 32 GB CF card instead.
This has the added benefit of not only being faster but larger as well. The original HD was 20 GB.