As far as copying the HD, use Carbon Copy Cloner (availiable on macupdate.com)
Is the Maxtor in a firewire or USB case? Is that what you mean by external? If so, you have 2 options. Either use the drive as it is, connected via firewire or USB, and continue to use the 10gb drive as boot, or you can take the drive out of the enclosure and put it inside the G3 case. The ribbon connecting the 10gb to the motherboard will have a place for it. The one difficulty with the old G3s is that you need to be aware of master/slave configurations, so you will need to set the jumpers on one of the drives to slave. I'm not confident as to where they go, so you may have to see if theres any info on the Maxtor website. I would also look at the B&W G3 forums on apple.com; those folks will know way more than I do. Hope this helps 🙂