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How to transfer files between two computers with Thunderbolt

Connect the two computers with a Thunderbolt cable and they get a private network link between them. On a Mac it’s called Thunderbolt Bridge. You then share a folder over it with File Sharing, share a whole disk, or move everything to a new Mac with Migration Assistant.

What you need

  • Thunderbolt 3 or newer, or USB4, on both computers.
  • A Thunderbolt or USB4 cable. A 40 Gbps cable is best.
  • The user name and password of an account on the computer you’re copying from.

Why this is harder than it should be

  • The cable only gives you a network. Nothing appears in Finder when you plug in. You turn on file sharing on one computer and connect to it from the other.
  • You need an account and password. File Sharing asks you to sign in as a user of the computer with the files.
  • You type an address by hand. There’s no router on a cable, so each computer gives itself an address starting with 169.254, and you look it up and type it.
  • The copy can go over Wi-Fi instead. If you connect by name while both computers are on Wi-Fi, the name can find the Wi-Fi address.
  • It doesn’t reconnect. After sleep, a restart or unplugging, you connect and sign in again, and the address may have changed.
  • Folders of small files are slow. File sharing handles each file with separate requests, so thousands of small files take minutes, not seconds.

Mac to Mac with File Sharing

Connect the cable first. Then set up sharing on the Mac that has the files, and connect from the other Mac.

Turn on File Sharing

On the Mac with the files:

  1. Open System Settings, choose General, then Sharing, and turn on File Sharing.
  2. Click the info button next to File Sharing. Under Shared Folders, click the add button and choose the folder to share.
  3. Under Users, check that your account has Read & Write access, then click Done.

You sign in from the other Mac as a user of this Mac. If you’d rather not use your own account, create a separate one in System Settings under Users & Groups and give it access to the shared folder.

Find the Mac’s cable address

Still on the Mac with the files:

  1. In System Settings, choose Network, then Thunderbolt Bridge.
  2. Click Details, then TCP/IP, and note the IP address. It starts with 169.254.

A yellow dot and “Self-assigned IP” next to Thunderbolt Bridge are normal for a cable link. If Thunderbolt Bridge says it isn’t connected, one of the ports or the cable doesn’t carry Thunderbolt or USB4.

Connect from the other Mac

On the Mac that receives the files:

  1. In Finder, choose Go, then Connect to Server.
  2. Type smb:// followed by the first Mac’s address, for example smb://169.254.10.20, and click Connect.
  3. Choose Registered User and enter the first Mac’s user name and password, then click Connect.
  4. Select the shared folder and click OK. It opens in Finder and appears under Locations in the sidebar.

Connect by the 169.254 address rather than by clicking the Mac’s name under Network. If both Macs are also on Wi-Fi, the name can find the Wi-Fi address, and the copy runs over the air without any warning.

Reconnect after sleep, restart or unplugging

The shared folder doesn’t come back by itself after either Mac sleeps or restarts, or after the cable comes out. Finder says the server connection was interrupted.

Connect again the same way. Connect to Server lists recent servers, but if the first Mac’s 169.254 address has changed, look up the new one first. For ways to keep a share connected, see shared folder disconnects after sleep.

Share a whole Mac with Share Disk

On a Mac with Apple silicon, Share Disk mode shares its entire disk without signing in. It’s useful when one Mac won’t start up normally.

  1. Shut down the Mac with the files, then press and hold the power button until “Loading startup options” appears.
  2. Click Options, then Continue.
  3. Choose Utilities, then Share Disk. Select the disk and click Start Sharing.
  4. On the other Mac, open Finder, click Network in the sidebar, double-click the shared Mac, and connect as Guest.

The shared Mac can’t be used while its disk is shared.

Moving to a new Mac

To copy everything, including apps, accounts and settings, connect the two Macs with the cable and open Migration Assistant on both. It uses the Thunderbolt link automatically.

Mac and Linux

Linux creates a thunderbolt0 network connection when the cable is connected, using the thunderbolt-net driver. Share a folder from the Mac as above, then connect from Linux.

Give the Linux side an address

To see whether thunderbolt0 has an address, run this in a terminal:

ip -4 addr show thunderbolt0

If the output has an inet 169.254 line, the link is ready. If it has no inet line, the computer is waiting for an address that never comes, because there’s no router on the cable. In GNOME:

  1. Open Settings and choose Network.
  2. Click the settings button next to the cable’s wired connection.
  3. On the IPv4 tab, choose Link-Local Only and click Apply.

Connect from Linux

In Files, open Other Locations or Network in the sidebar, depending on the version. Type smb:// and the Mac’s 169.254 address in the Connect to Server box, then sign in with the Mac’s user name and password. Other file managers have a similar option. You can also copy with rsync or scp over the link if you turn on Remote Login on the Mac.

In our tests a Linux laptop and a Mac connected on one 20 Gb/s lane of a 40 Gbps cable, rather than two. The link itself carried 16.3–17.8 Gb/s, so copies can’t go faster than that.

Why file sharing over Thunderbolt runs below the cable’s speed

A 40 Gbps cable rarely delivers 40 Gb/s of file copies with the sharing built into macOS and Windows. Some limits come from the link, but most come from the file-sharing protocol, SMB, and from Mac and Windows not being tuned for each other.

The link

  • One lane or two: 40 Gb/s needs both computers to bring up two lanes. Some pairs bring up one, which gives 20 Gb/s each way.
  • Uneven directions: in our tests, a Windows laptop could send only 12.0 Gb/s to a Mac over a 40 Gb/s link, while the Mac sent 24.2 Gb/s back, before any file sharing was involved.
  • Drives: a copy can’t be faster than the receiving drive can write.

SMB file sharing

  • One file at a time: SMB opens, writes, checks and closes each file with separate back-and-forth requests. That’s fine for one large file, but a folder of thousands of small files spends most of its time waiting. In our tests a Mac took almost four minutes to copy 20,000 small files to a Linux computer over SMB on a 10 Gb/s link.
  • Browsing large folders: Finder and File Explorer ask for details, icons and previews file by file, so opening a big shared folder can take seconds. Listing 20,000 files over SMB took 42 seconds in our tests.
  • Signing: recent versions of Windows 11 require SMB signing by default, which adds work to every request.

Mac and Windows together

  • Blocked by default: Windows treats the new cable connection as a Public network, where file sharing is switched off, until you change it to Private network.
  • The wrong connection: connecting by the other computer’s name can find its Wi-Fi address instead, so the copy quietly runs over Wi-Fi. Connecting by address works only until the address changes.
  • Addresses change: the cable link uses self-assigned addresses, which can change when you unplug and reconnect, so a saved connection stops working.
  • File names and hidden files: a Mac allows characters in file names that Windows doesn’t, and Finder leaves hidden .DS_Store files in shared Windows folders.
  • No reconnection: after sleep, a restart or unplugging, you connect and sign in again.

The full steps for a PC are in transferring files between Windows and Mac. Our test results have the measurements.