That was odd. Connected the Seagate GoFlex Thunderbolt drive adaptor fitted with a 250 GB HD to my MacBook Air, intending to do a backup, but it wouldn't mount. System Information showed the drive adaptor as "Unknown Device." Trying other drives on the adaptor didn't help, nor did a restart. "Maybe the T/bolt cable is faulty," thinks I. So I started up the MB Pro in target disk mode, connecting it to the MB Air via the same T/bolt cable. The MB Pro's SSD was mounted, so the cable was OK.
Now I tried a diagnostic SSD (TechTool Pro-created ProToGo drive.) - it mounted OK & System Info correctly identified the adaptor as a GoFlex. Ejected the ProToGo & attached the backup clone. It mounted & SuperDuper! app commenced the incremental backup as expected.
Me no understand but me happy as the price of T/bolt GoFlex interfaces is now ridiculous. The cheapest I've seen them now they're a discontinued item is US$280, crazy, I paid AU$135 for it when new.