Migrating from 2010 iMac to M4 Mac Mini

If you have not already done so, create a Time Machine backup of the old iMac on an external drive. Use a drive that is 2x - 3x the size of the storage you will configure the new Mac mini with.

When you get your new Mac, connect the Time Machine backup drive (that you made with the old iMac) to the mini before you first turn it on to set it up. When you are asked if you want to migrate your users and data from another Mac, select the Time Machine drive. When (if) you are asked if you want to 'inherit the backup' of the old iMac, select yes. Your new Mac mini will continue to use the same TM backup drive as its own, including the files that were held on the old Mac.

This will be the easiest, fastest way to move your stuff from the old to the new Mac. You only want to move your user files, though, not any of the system settings or applications.

Back up your Mac with Time Machine - Apple Support

Either or both users can be an admin account.

·       Always connect your old Mac's Time Machine External Hard Disk prior to startup, this will allow a migration from your TM backup and it will be dead simple! When you start the new Mac for the first time it will startup Setup Assistant (SA), SA will get to a point where it asks if you are migrating from another computer, select Yes and follow the on-screen prompts and it's done! It is that simple! This is the easiest, quickest and most reliable method to migrate from the old Mac to the new Mac is to simply connect the old Mac's Time Machine External Hard Disk to the new Mac using a USB cable before you turn on the new Mac. 

·       NOTE 1: You will likely need a USB A to USB C adapter, you can find them on Amazon for $10 or less.

·       NOTE 2: If you have already done SA on the new computer then simply connect the Time Machine External Hard Disk using a USB cable and launch Migration Assistant (MA) which is located in Applications - Utilities - MA. Then follow the on-screen prompts.

·       The ONLY difference between MA and SA is MA will create a new user account that you need to log into.

Both SA and MA migrate applications (not 32 bit apps), data files & settings. They do not migrate Mac OS itself. Some professional apps such as Adobe and Microsoft apps may require re-installing their apps as they will not allow them to be migrated. As long as you kept the security keys for installing their apps, it is not an issue. 

Finally, never EVER use Wi-Fi when doing Setup Assistant or Migration Assistant. Why, it simply fails in most cases. Please read and follow the directions in Setup Assistant and Migration Assistant. If you do not have a Time Machine backup of the older Mac then create one! If you are not aware of Time Machine, please read Back up your Mac with Time Machine and finally if you need an External Hard Disk to use as a Time Machine External Hard Disk then many experience users on these forums prefer the OWC Mercury Elite Pro due to it's reasonable price, high level of quality and ease of use. 

Personally, with a Mac that old along with an old OS, I would do fresh install of all apps with the latest versions (note that many of the apps compatible with High Sierra will not work on the new Mini including most all the Apple apps).

Then manually migrate user data as needed because after installing latest apps on the new Mini many user preferences, caches, and other app specific data may no longer be compatible with the new apps.

This process while seeming taking longer will likely be less trouble than trying to sort out all the incompatibilities of migrating such an old system. Basically like cleaning out the attic, garage or basement when moving to a new home.

Just to add, using this approach you can also eliminate a lot of stuff that you haven't used or needed that has amassed on your old computer.