Migration Assistant is the easiest way to transplant your heart and soul from one Mac to another. While it used to be fragile and often failed or required more attempts, I have found it more and more reliable in recent years. I have used it a few times in the last few months and it worked almost perfectly.
Migration Assistant has improved the number of ways it lets you connect two Macs, and has become better at using the fastest connection method. On newer Macs, the best way will always be Thunderbolt-to-Thunderbolt. This requires a Thunderbolt data cable. For a slightly worse, but not terrible performance, you can instead use a USB 3.1 or 3.2 cable with USB Type-A or USB-C at both ends. (If you are not sure, see this guide to find out what kind of USB-C cable you have.)
If you have an Ethernet port on both devices, you can connect a standard Ethernet cable, but it will only provide 1 Gbps compared to the potential 10 to 40 Gbps for USB 3.1 or Thunderbolt 3.
After going through the initial steps on each Mac to launch Migration Assistant and validate one Mac to another and begin the transfer, Migration Assistant displays the selected network on the machine that copies as “Current Connection” at the bottom of the screen.
If this is not the method you prefer or you would like more information, you can click on the Link Details link in the lower right corner. This brings up a dialog showing the performance of all available methods. In the figure, I migrated my wife’s data from one laptop to another; Thunderbolt 3 started correctly with a Thunderbolt 3 to Thunderbolt 3 cable.
Migration Assistants reported speeds of 192MBps (1.5 Gbps) not only seemed phenomenal, but the transfer of hundreds of gigabytes took about 30 minutes.
macOS reported the other two available methods as bad, showing a yellow dot to the right instead of a green one, as next to the Thunderbolt input at the top. A peer-to-peer network that uses a special direct Wi-Fi mode between the two Macs would have been over ten times slower. It was fabulous compared to the very low data rate of 1MBps (8Mbps) reported over Wi-Fi, probably because one or both machines were connected to a remote Wi-Fi gateway instead of the one just meters away.
If you do not like the chosen method, you can cancel the migration and test cables, change the Wi-Fi network in the setup phase or buy a new cable to speed things up.
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