Only one of two Time Machine hard disks seen on new iMac

I just bought a new iMac and I successfully transferred everything from the old computer to the new one. I used two portable external hard disks with Time Machine to back up the old Mac but only one of them visible when I connect them to the new Mac. Only one is visible on Disk Utility as well. Both are connected to the new Mac using a USB-A to USB-C adaptor. Neither has a separate power source.

Any idea what the problems is?