How to copy a Time Machine backup
Cloning a Time Machine backup from one external hard drive to another is surprisingly difficult.
First – what didn’t work:
Using drag-and-drop in Finder
This is what Apple recommends in their support article Time Machine: How to transfer backups from a current backup drive to a new backup drive. But the next day Finder was still on “Preparing to copy”. And it hadn’t processed anywhere close to the about 27 million files in my 2 TB backup. Some searching revealed this is a common problem, where it will either never finish – or if it does the copy will not be usable in Time Machine.
Restoring partition in Disk Utility
This gave the error message “Source volume format on device ‘/dev/disk3s2’ is not valid for restoring”. It is a standard Mac OS Extended (Journaled) partition created by Time Machine. First aid in Disk Utility finds no problems.
Using Carbon Copy Cloner
But SuperDuper managed it (in a little over 3 days) – using the default settings (Backup – all files). And it’s free for basic functionality.