You can tell the really serious modeller – no matter whether they build model ships, trains, or aircraft – because they don’t buy kits. Instead they build using the raw materials, and fabricate their models from scratch.
According to the Oxford English Dictionary, the phrase from scratch is a figurative use of the term used to describe the starting point of a competitor in a handicap race, who receives no odds or benefits from handicap. Hence doing something from scratch starts from a position of no advantage, from nothing.
When you replace an old Mac with new, there are two paths open to you: you can migrate your Documents, apps, settings, etc., from the old to the new, or you can build the new Mac from scratch.
The latter means that you install everything afresh, using the latest installers. For a small system, or for those with a lot of time on their hands, building from scratch is ideal, as no old cruft gets pulled across, and you only install what you really want and need.
Few of us have time to build a new Mac from scratch. It is bad enough having to lose a few hours to migrate from our last system. But sometimes time saved commissioning the new Mac will be lost later on, sorting its problems out.
Migration looks deceptively simple: ensure that your old Mac is thoroughly up to date, de-authorise its apps as necessary, then hook it up via a high-speed connection to the new Mac. You can also migrate from a mirror copy of its drive, made using Carbon Copy Cloner or similar, or from its Time Machine backup.
If you only elect to migrate a limited amount of material, such as your Home folder and personal settings, that compromise will leave you almost as much work as a scratchbuild. Most who opt for migration do so for the lot.
Even in these days of strict kernel extension security, with SIP and its related security mechanisms, this does not stop old and unstable extensions (KEXTs) from making their way onto your new Mac. In theory you should have cleaned those off your old Mac years ago, or perhaps in a previous migration. But until they tip a Mac into serious instability, most of us leave them alone, together with their LaunchAgents and LaunchDaemons which can also do terrible things to newer releases of OS X.
These low-level components can also be hard to identify. You may end up having to search on com.squigglebum.hyksos.kext to work out what it is, and whose support service to ask how to uninstall everything properly. I have recently explained how to perform manual uninstallation, but it takes time and can be quite messy.
Migrating App Store apps means that you will have to go through the process of authorising your new Mac to use them, but that is far quicker and simpler than doing that and installing each from the App Store. As I have recently discovered, some App Store apps such as OS X Server and Xcode seem to have to be installed afresh in any case.
You will also have the problem of restoring broken links. These can appear almost anywhere, but iTunes Music libraries appear particularly prone, as detailed here.
Then there will be many folders full of old documents which you should have archived years ago, but just migrate each time that you replace your Mac. Unless you are one of the very few supremely organised Mac users who does operate an archiving system – rather than just thinking about it wishfully – some of those documents could go right back to your first Mac.
You are also bound to have a lot of old apps, which either won’t run any more, or you have forgotten what they did. Order your Applications folder by date, and see if you can beat my oldest items: an app folder last modified on 27 October 2004, and an app from 2 May 2007! And no, I cannot recall what the latter does.
Maybe when I replace this iMac, I should build my next startup drive from scratch.