I re-cloned the cloned ATA drive to a new SATA drive,
and the G4 now starts up perfectly.
That ATA clone takes a few minutes to appear on the
desktop, and often Disk Utility cannot locate it until
after quite a long time of searching.
But I'm still getting the "Disk unreadable" warning after
start-up from the new SATA drive.
Sounds like I need to replace/remove the "disappeared" ATA
as well as the original cloned ATA drives.(??)
Will do so today, but meantime at least the machine
is working normally at start-up.
It was the start-up disk. I sourced a pair of NOS ATAs
and whacked them in. Everything is fine now.
Incidentally, the two original ATAs I bought on the
same day from the same vendor. Exactly two years
ago this month. One was WD, the other Seagate (the last two the dealer had).
They both failed just 48 hours apart from each other!
Thanks to other posters for their tips.