Besides Thunderbolt, two Macs joined by a USB‑C cable can talk over the cable’s USB wires. When the cable doesn’t come up as Thunderbolt, because it’s a charging cable or a port has no Thunderbolt, Second Display uses that USB link rather than leave you without a picture. We’ve tested this between Macs with Apple silicon.
- The picture is compressed, as over Wi‑Fi, at the quality the speed test recommends. Highest needs Thunderbolt.
- The menu names the link USB, for example “USB · Medium quality”, and adds: “On USB for now. For Thunderbolt, try a ⚡︎ port; the cable may be a USB‑C charging cable.”
- It works even when Allow Wi‑Fi and Ethernet is off: a cable is a cable.
- As soon as the cable comes up as Thunderbolt, for example in a ⚡︎ port, the picture moves over to Thunderbolt by itself.
- A Mac laptop may ask whether to allow the accessory to connect. Click Allow.
During setup
The Cable step looks for Thunderbolt, the link of the lossless picture. With a cable that isn’t Thunderbolt it says “This looks like a USB‑C charging cable, or the cable is in a port without the ⚡︎ symbol.” Its tip “A USB‑C charging cable will not work.” means that such a cable can’t carry the lossless picture.
To go on with the cable you have, choose Use Wi‑Fi or Ethernet instead: the next step still finds the other Mac over the cable, and the picture goes over USB, compressed.