Solo FireWire e Thunderbolt supportano la modalità disco di destinazione.
Devi rimuovere il disco interno e inserirlo in una custodia esterna con interfaccia UltraATA.
In alternativa puoi creare una pen drive avviabile (MacOS 9.1 o superiore), avviare da essa e creare un'immagine del tuo HDD con Disk Copy
e salvarla in una condivisione di rete. La creazione di una pen drive avviabile e l'avvio da essa potrebbe richiedere un po 'di tempo a causa della bassa larghezza di banda di USB1.1 (12 MBit / sec ≈ max. 1MByte / s).
In base a questo articolo Mac OS X 10.3.9 - l'ultimo Mac OS X che supporta il G3 - sfortunatamente non fornisce l'avvio da USB:
Further refinement I just came across in my notes, on the system requirements to boot from Macintosh USB ports: OS 8.6 through 9.2.2, and OS 10.4.3 and later. OS 8.6 - 9.2.2 might need the last of whichever versions of the USB extensions that each of these System versions supports (for OS 9.1 through 9.2.2, USB 1.5.6 is the preferred version; OS 9.1 and later don't need the extension "USB Mass Storage Support", especially since it sometimes freezes Macs running OS 9.1 - 9.2.2 that are trying to boot from USB), though other details as to the proper combination and versions of USB extensions under OS 8.6 - 9.2.2 is screwy, and too lengthy to post here, especially since we're dealing with OS X anyway.
The first Macs to support USB booting, are the first slot-loading iMacs (including the 350 MHz model with no Firewire ports), and the Firewire Powerbook G3. I've even booted OS 10.4.11, from a USB 2.0 flash drive, on a Mirrored Drive Doors G4, from a USB 2.0 PCI slot card, with no special tricks, Open Firmware modifications, etc.