I used the USB3 on the EXHD with an adapter plugged into USB-C on the back of the iMac - if that makes any difference regarding speed.
(
WARNING: this may be more than you want to know but here it goes anyway.)
It does make a significant difference but perhaps not quite in the way you envision. USB 3 is a data communications protocol while USB C is a type of connector and can be used for either USB or Thunderbolt 3 data communications. On top of that USB 3 covers three different protocols four or five marketing names and speee ranges from 5 Gbps to 20 Gbps.
This Wikipedia Article does a good job of detangling the relationship between USB protocols and connectors.
According to Apple's specs the iMac has
- 4 Type A USB 3.0 ports (Up to 5 Gbps)
- 2 USB C ports capable of
- Thunderbolt 3 (40 Gbps)
- USB 3.1 (Up to 10 Gbps)
- Display Port (with adaptor)
- Thunderbolt 2, HDMI, DVI, and VGA (with appropriate adaptors
So if your eternal drive is connected via one of the USB type A connectors the best speed you can get is 5 Gbps, but if you are plugged in through a USB C connector you could get up to 10 Gbps. But since the speed is limited by the slowest device the USB port on the disk drive could be the limiting factor and I have no idea if it is USB 2.0, 3.0, 3.1, or 3.1 Gen 2 (a.k.a. 3.2).
To be perfect honest USB 3.0 (5 Gbps) is generally fast enough for Time Machine backups. The only time you are likely to be aware of the speed is during an initial backup or a full system restore.
