Apple eMac 1 GHz, G4 - extra hard drive, what, how?

As an idea- purchase a SATA-compatible external drive enclosure which supports FireWire (Which is in effect a SATA-FW adapter). That is the only easy way to add extra storage to eMac G4s (Which are fine machines btw).

first the emac has only ide connectors inside NO SATA
firewire is better and faster so get a external firewire enclosure cheapest and less work intense way
other option is a bit complicated get a bigger hardrive for internal , dont worry about your data keep the old one where it is for the moment

needs a bit of taking eMac apart ,
but first install carboncopy cloner
after you took the emac apart , take the optical drive out fit your new BIGGER harddrive inside the optical drive bay (fix temprary as it wil come out again and make it master with the jumpers)
now put the emac back together (no need for all outside screws ) switch it on and it should see this big hardrive in finder , use disc utility and format it extended journaled ( make partitions like you want )
now copy with carbon copy cloner all data ( bootable)to the new big harddrive
done
now take the emac apart again fit the big one in place of your full small one and fit the optical drive again , everything back together , plug it in and be happy with your big drive