Time Machine: inherit a backup from another computer

EDIT FOR MACOS BIG SUR:

Apparently the -a options of the tmutil command does not work in BigSur. Luckily somebody found a solution:

for f in <youBackupComputer>*; do if [[ -d "$f" ]]; then sudo tmutil associatedisk <theNewDisk>"$f"<theOldDisk>; fi; done

In my case, the command looked like this:

for f in /Volumes/Time\ Machine/Backups.backupdb/MacBook\ Pro\ A/*; do if [[ -d "$f" ]]; then sudo tmutil associatedisk /Volumes/Macintosh\ HD "$f"/Macintosh\ HD; fi; done

After this, Time Machine copied just 30.94GB of data :)

Today I migrated my information to a new computer. Of course, I have an external hard disk with a Time Machine backup on it. In order to be able to use it in the new computer, first I knew I had to perform some additional steps. I found them here, and the first is:

sudo tmutil inheritbackup /Volumes/YourDiskName/Backups.backupdb/YourOldComputerName

Perfect. I opened the terminal an run the command. The output was the following:

Unexpectedly found no machine directories for “/Volumes/Time Machine” during collision check.

There is not much information about this error (you may check here). I discovered that the problem is that the terminal, due to privacy policies, can not access the Time Machine backups data. In order to change this you have to go to the “Security & Privacy” configuration, and under the “Privacy” tab, allow “Full Disk Access” to the terminal app:

After doing that, the command worked, and Time Machine did not need to copy all the information of the new computer, but just the differences between the new and the old one: they are very similar, which makes the process faster, reduces disk usage and allows me to keep the old and new backups in the same backup disk.

I also had to link the old disk where MacOS was stored (/Volumes/Time Machine/Backups.backupdb/MBP-A/2018-11-01-152028/MAC OSX ) with the new one (/Volumes/MAC OSX ):

In my case, instead of copying again 323GB of data, it only needed to copy 55GB: