I've been using two different external drives with Time Machine for my 2016 MacBook Pro running Catalina 10.15.7 but beginning two weeks ago, with both external drives, Time Machine gets stuck on "preparing backup." I've let it run overnight and still it can't get past that "preparing backup" stage.
The preparing backup stage is mostly an examination of the SOURCE data. You should run Disk Utility First Aid on your source drives.
However, if the amount of free space on the Backup drive is too small, it may be doing a consolidation (or multiple consolidations) in an attempt to make enough space available. If that is the case, there are no know manual steps that can be taken to hurry it along -- it needs to keep consolidating until there is stuff so old, it can be discarded and more space will be created.
It is important to understand that Time machine can not just discard you oldest backups, it must methodically consolidate them. Old backups contain a certain number of old files that have Not changed, so the original copy of that file is being referenced and re-used in dozens of more recent backups as well.
The preparing backup stage is mostly an examination of the SOURCE data. You should run Disk Utility First Aid on your source drives.
However, if the amount of free space on the Backup drive is too small, it may be doing a consolidation (or multiple consolidations) in an attempt to make enough space available. If that is the case, there are no know manual steps that can be taken to hurry it along -- it needs to keep consolidating until there is stuff so old, it can be discarded and more space will be created.
It is important to understand that Time machine can not just discard you oldest backups, it must methodically consolidate them. Old backups contain a certain number of old files that have Not changed, so the original copy of that file is being referenced and re-used in dozens of more recent backups as well.
Time Machine stuck on "preparing backup"
