I had a similar experience trying to transfer data from a backup hard drive to a new MacBook Pro. I bought an Apple USB-C to USB adaptor.
When I tried to connect directly to the hard drive, it resulted in a message saying the hard drive was not repairable.
When I connected the MacBook to a router using Wi-Fi, and the hard drive to the router using a USB cable, I was able to initiate the transfer using Migration Assistant. However, this continued to fail, and it usually took about 2 hours, but sometimes it failed rather quickly. After two trips to the Apple Store and multiple, long conversations with puzzled Apple experts on the phone, I stumbled across the solution.
I did a clean install of the new machine, under Apple expert phone guidance. Then I bought a Belkin USB-C to Ethernet adaptor at the Apple Store. I connected the new computer to the router via USB-C to Ethernet and left the hard drive connected to the router using USB. All the user data was transferred in about 2 hours, much faster than the projected Wi-Fi method.
I really missed the Ethernet connection on the new MacBook, which would have made this transfer easy.