SuperDuper app was taking far too long to perform a regular clone of my iMac’s Fusion drive (156 GB in use out of 1.03 TB). Killed that process & changed the settings so it wouldn’t start backups as soon as the drive is detected.
Now backing up from the same source to the same destination using Carbon Copy Cloner, much more progress. The ext drive is an old USB 2·0 case although with a trio of dongles I could connect it via FW 800.
Instead, I have ordered a 3.5-in USB 3 drive enclosure from Amazon.