Please refer to the following screenshot to create a new backup:

1. Clicking "+Add Backup".
2.Clicking "Deselect All", then turn to "Select All".
3.Clicking "Customise Selection".
4.Select the files you want to backup in HDD A here.
5.Clicking "External Drive" and select "HDD B" under it.
6.Select the backup type you want.
7.Clicking "Back Up Now" in the end.
Thanks for your supporting of Nero!
Nero Support Team