How to backup data from one external drive to another (one-way)
I've just bought another 5TB external disk (Seagate backup plus portable), where I always would like to have a 1-1 copy of the whole date from the first disk.


It’s not a sound strategy to back up a backup.

I write about this in much more detail in another related answer, but to summarize here, you should have distinct backup jobs, not copies of existing backups.

Having an “identical copy” of a drive is good for redundancy and the correct tool for that job would be a product that implements RAID where the drives are mirrored.  Manually (meaning non-RAID) syncing drives falls short of being a good backup solution because if there are issues with the original backup (i.e. file corruption), the same issue is replicated to the other drive making the backup useless.

Syncing is a tool best used when you need to have files accessible in/from different locations and are typically not being simultaneously accessed (changed).

What you should do with your current setup is continue to use the MyBook for your daily backup and then create a new backup job that does a differential (only what has changed) backup when you plug in the Seagate.