Hi there. I think you are thinking this process is harder than it really is. Here is what I suggest. Use Migration Assistant to do the transfer. Migration Assistant is a program provided by Apple for transferring your files from your old computer to your new computer. It is very easy to use. Be sure to read and follow its instructions. Basically, the process is as follows.
You first begin by starting the process of setting up your new computer. At some point you will be asked if you wish to transfer files. Respond YES and follow the instructions you will be given.
You will be told to turn both computers on and activate Migration Assistant in the old computer.
The computers should be connected with a USB cable to facilitate the transfer. Wi-Fi is simply not suited for a transfer of so much date and corruption often results. Do not use it. It can corrupt or loose data.
You will be given several options as to what to transfer. You will likely want to transfer your settings and preferences.
You can transfer all or none of your files. You do not have an option to select certain files to transfer. If you wish to do that, then do so manually.
The old computer's operating system will not be transferred. No data from the old computer is erased.
Allow for up to an hour for the transfer process to complete once you start it.
It would normally set up the new computer the same as the old one. That said, to do what you want to do, I would do the following. First manually transfer the extensive files from the old computer to the new external disk. Migration Assistant can not do that as it only transfers files to a disk with an operating system on it. Then, since you do not want to transfer the files to the smaller internal HD on the new computer, do not select the option to transfer any files and you should get what you want. Easily done.
Enjoy your new Mac.
I hope this is helpful. Good luck.
