Preparing to migrate to a new Mac
I am planning on buying a new iMac, presumably running High Sierra. How can I best move apps, documents, and data (such as saved passwords) from my current Sierra system to the new iMac?
Before your new Mac arrives, have a thorough clear out of the apps and personal files stored on your old Mac, removing anything that you no longer need. Ensure there aren’t any old extensions or other software left behind that might not be fully compatible with High Sierra. Note that some apps, such as Adobe CC, must be deauthorized on the old Mac and then reauthorized on the new one; anticipate the apps that may be affected so you can work this out early.
When you set up your new Mac, ensure it’s on a high-speed network connection with your old one. For a really quick transfer, you might be able to connect both machines directly using target disk mode over Thunderbolt; start your old Mac with the T key held down to effectively make its internal storage an external drive to your new Mac.
When you start up and run through the Setup Assistant on your new Mac, you’ll be invited to migrate your apps, documents, and settings from the old system. Accept the invitation, and select what you want to migrate. If you don’t want to do it all at once, you can always run Migration Assistant at a later date to bring across whatever you leave out.
Migration can take a very long time — sometimes a day or more — depending on how much there is to copy across, so a fast connection between the two computers is important. You also won’t be able to use either Mac until it’s complete.
You can connect two Macs using Thunderbolt and access the contents of the old Mac from the new one by starting it up in target disk mode.