Thunderbolt Bridge Not Working: 9 Causes and Their Fixes
Thunderbolt Bridge is reliable — when everything is in place. When it isn't, macOS says very little. Here are the causes, in the order we meet them at support, each with the test that confirms it.
1. The cable isn't Thunderbolt / USB4
Symptom: in System Settings › Network, "Thunderbolt Bridge" stays "Not connected" on both Macs. Cause: a USB‑C charging or USB 3 cable. Only Thunderbolt 3/4 (⚡ logo) and 40 Gbit/s USB4 cables create a link between two computers. Fix: change the cable. Cable guide.
2. A hub or dock sits between the Macs
Symptom: same as above, even with a good cable. Fix: connect the Macs directly, port to port.
3. The port isn't Thunderbolt
On Intel iMacs and some Mac minis, only the ports marked ⚡ are Thunderbolt; the other USB‑C ports aren't. On M1/M2 MacBook Airs both ports are. Try the other port.
4. Thunderbolt Bridge is missing or disabled
Symptom: no "Thunderbolt Bridge" interface in the list of network services. Fix: System Settings › Network › "…" menu (bottom) › Add Service › Thunderbolt Bridge. On a Mac with no Thunderbolt port (a 2019 iMac with USB‑C only, say), the service doesn't exist.
5. Connected, but the other Mac stays invisible
Symptom: the bridge shows "Connected" or "Self‑assigned IP", but nothing appears in Finder › Network or in DiskBridge. Possible causes:
- File Sharing isn't on on the other Mac (System Settings › General › Sharing). DiskBridge checks this in its welcome window.
- On macOS 15 and later, the Local Network permission was denied to the app (System Settings › Privacy & Security › Local Network). DiskBridge says so clearly and offers the button to fix it.
- A third‑party firewall blocks port 445 (SMB) or Bonjour (5353). macOS's own firewall lets File Sharing through.
- Two Macs both running Internet Sharing on the Thunderbolt Bridge: they fight over the DHCP server role and end up on different subnets. Leave Internet Sharing on only one Mac (DiskBridge flags this in its diagnostics).
6. "Self‑assigned IP": is that normal?
Yes. With no DHCP server, macOS assigns a 169.254.x.x address to each end; the two Macs still find each other through Bonjour. It's only a problem if you type the address by hand and it has changed.
7. Password refused
File Sharing asks for the password of your account on the other Mac (not this Mac's), and the account must exist on that Mac. With a standard account, only explicitly shared folders are visible; an administrator account sees all volumes.
8. It works, but it's slow
The link runs at 10‑40 Gbit/s; if you copy at 100 MB/s, the link isn't the problem. Check: a spinning hard drive on one side? a long uncertified cable (see the cable guide)? a copy that actually goes over Wi‑Fi because you connected by the Mac's name rather than through the bridge? In that last case, turn Wi‑Fi off for a moment: if the copy continues, it's on the cable. DiskBridge always connects through the bridge address, never over Wi‑Fi.
9. Volumes vanish or the Finder hangs on unplug
They were unplugged without being ejected. Always eject before pulling the cable — or let DiskBridge do it: it watches the link and unmounts the volumes cleanly when the cable is removed.
One‑command diagnosis
In Terminal: ifconfig bridge0 — the line status: active confirms the physical link is up; inactive means cable or port. DiskBridge shows the same thing, in plain words, in its menu › Diagnostics…, along with the state of File Sharing, the Local Network permission and the other Mac.
If none of this helps, write to info@dronetools.net with the diagnostics text: we answer quickly.
Get DiskBridge on the Mac App Store
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