Performing first clone of iMac, it's going to a USB 2·0 HD. Amazon confirms they have shipped its replacement USB 3·0 interface enclosure, but it's two weeks out since I opted for the cheapest delivery method. Also doing first Time machine backup, to a 3 TB USB 3·0 ext drive. I'm using an in-between* ext HD for storage of large items: it connects via Thunderbolt 3 to Thunderbolt 2 adaptor plus a Thunderbolt 1 to FireWire 800 adaptor.
*In-between as in faster than USB 2·0 but slower than USB 3·0.