I maintain several Mac computers for a local school. One of my duties in this position is to migrate a persons’ user account from an older Mac to a new Mac. I use the excellent Migration Assistant App provided by Apple for this purpose. Migration Assistant has been around on the macOS for many years. It has gotten better with time. However, there are still occasional issues where the migration process will not work.
The one problem that I constantly run into is migrating from an older Mac running an older version of the macOS, like High Sierra for example. When I try to migrate these machines to a new Mac, the migration process stalls at “Calculating”. Never fear, folks, there is a solution to this problem.
I have found the way to alleviate this issue is to bring the older Mac forward to the most current version of macOS it can run. If you can get that machine up to macOS Catalina, Big Sur or even Monterey, the migration process works every time. I just completed migrating a 2017 MacBook Air. The Air was running macOS Mojave. On my initial attempt, the migration process would not complete, but it did work after updating the older Mac to macOS Monterey.
If you are trying to migrate to an older Mac, I recommend updating the operating system before you start. Yes, you can try the migration without the update, and it might work. But, it is just easier to take a little extra time to update the older machine and then do your migration.
One more thing here. If you update the older machine to the most current version of macOS, the new machine may not be updated to that version yet. These new Macs sit on a shelf until purchased, sometimes the macOS is not updating to the most current version. If that is the case, you will get an error stating you cannot do the migration because the OS on the older machine is newer. So, while you are updating the older machine, make sure the new one is totally up to date.
I hope this helps your migration efforts.