The right cable for Migration Assistant over Thunderbolt with Target Disk Mode
So yesterday it took me a while to figure out how to use Target Disk Mode over Thunderbolt to transfer data from an older MacBookPro to a new one. So I thought I would write it down for the next person figuring this out.
You will need a proper Thunderbolt 3 to Thunderbolt 3 cable. Apples doesn't sell one. Just having a cable that has USB-C plugs on both ends doesn't work.
If the old Mac has Thunderbolt 2 you will need a Thunderbolt 2 to 3 adapter, which Apple sells and a Thunderbolt 2 cable.
To boot in Target Disk Mode shut down your Mac. Then turn it on while holding the key t until you see a Thunderbolt icon on the screen.
Start Migration Assistant on the other Mac and connect both with the Thunderbolt cable.
Data should be completely copied over in a few hours at most.
MacBook Pro 16", macOS 10.15