Backup Can't be Started. Plug in the External Files of this Backup

This is not an error message that I have seen before - searching knowledge base did not bring up anything useful.

To resolve the issue we need to look at the log files; To do this @Steve Smith offers the following advice:

There is a new MVP Assistant log viewer tool that has now been made available by Acronis via the Community Tools page.. 

If you have Disks & Partitions backups created on ATI 2020 or later using .tibx files, then look in the Backup Worker logs.

If you have Files & Folders backups using .tib files (or Disk backups from earlier versions using .tib files) then look in the Demon logs.

Other logs are shown by the MVP Assistant under the 'Active Logs' heading of the Log Viewer page of the Assistant.

As you are using ATI 2019 you will need to look at the Demon logs. Please post the logs (copy them to a file and upload the file is the best way to go).

Edit: Please confirm that the new drives are attached to a SATA port, if not haw are they attached? Is the backup being run from within Windows or using the recovery media?

Ian 

Backup Can't be Started. Plug in the External Files of this Backup

The message above suggests that this backup task has been run previously and has created backup files, hence it is looking for those files but they are not found on the new, empty drive!

Do a validation for the backup task and click on either Ignore (if offered) or cancel for each message about missing files, then retry the task.