Where was the 500GB drive? In my external Firewire box, which is where I put the damn thing to test it a couple of weeks ago!
MUCH later - spoke too soon, for some reason Leopard install repeatedly wouldn't work, I think the DVD drive didn't like the disk which has had a LOT of use. I got Tiger on OK, so I'm now trying to make a bootable Leopard install disk on my Macbook Pro; managed to copy it to an image file, but OSX 10.11 disk utility apparently doesn't burn DVDs natively any more, which is a bit of a PITA. I'm trying to burn the file from Finder but I'm not sure that will work.
If that doesn't work I think I'll use the firewire drive box to make a bootable hard disk of it. You can't, unfortunately, boot a G4 box from USB, so a USB version is out.
Any other suggestions?
later still: DVD is bootable - hopefully will install.