Tried cloning via the other T/bolt cable: failed. Write errors. Now checking status of Thunderbolt port itself. I have a T/bolt-to-USB 3.0/Gigabit Ethernet adaptor available, so I’ve connected the drive via the reliable USB 3 SATA interface and the clone process is yet to report errors. So it seems the Seagate GoFlex Thunderbolt/SATA adaptor I bought about 5 years ago is fuct.
I’d bought it to use with both the 2011 MBA & MBP computers I used to operate. Now the new iMac & used (one previous owner -Apple) 2014 MBA have USB 3 ports (plus Thunderbolt 3 on the iMac) so I don’t t really need the T/bolt/SATA adaptor.
You can’t get them any more, or not for any reasonable price.