I realised there was one older Mac I hadn't tried with the recalcitrant iXpand drive, the 2008 15-in MacBook Pro. After being connected for about 5 minutes, it deigned to mount and I was able to save the 10 Gb of scans to another drive.
I wondered how to do this with both USB ports being used, one for the mouse (the trackpad is often unresponsive) and the Time Machine drive doing a much-needed backup.
Then I remembered I had a FireWire 800 to SATA bare drive adapter available. I used this with a spare SSD to copy the 10 GB of scans.