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Macs cannot technically connect to one another via a USB cable—the closest you can get to a USB-to-USB connection on a Mac is to run a cable from one computer's USB port to the other computer's Ethernet port.  USB-to-Ethernet connector: These connectors are universal, meaning you don't have to buy one specifically made for Apple computers. One end of the adapter has a male USB connector, the other a female RJ-45 port for the Ethernet cable. 10/100BASE-T Ethernet cable: This cable is standard, has RJ-45 connectors at either end, and can be purchased at any electronics store. See How to Transfer Files Between Two Macs for simpler ways  to get files from one computer to another. If only one of the computers has an Ethernet port, plug the USB adapter into that computer. Otherwise, it doesn't matter which one you plug into first. This port is likely on the side or back of your computer. The wiring is complete. On each computer, open the Apple menu, click “System Preferences,” and then select “Sharing.”  When the Sharing preferences appear on the screen, you'll also see the name of the computer you're on. It doesn't matter which computer you use to begin this process. On one computer, open Finder, select “Go,” then “Connect to server.” Click “Browse” to show a list of possible computers to connect to. When you see the name of the second computer appear in the results, double-click it, then enter your password (if prompted). You should now see a list of the files on the second computer from the first computer. Drag and drop files between computers within this Finder window.

Summary:
Gather the necessary accessories. Connect the USB adapter into a USB port on Computer 1. Plug one end of the Ethernet cable into the RJ-45 port of Computer 2. Plug the other end of the Ethernet cable (connected to Computer 2) into the USB adapter. Open the Sharing preferences on both computers. Use Finder on one computer to connect to the other. Copy files back and forth between computers.