Creating a backup does not threaten existing data on the (external) drive. Just will be added a few files in a new folder. If there would be enough space for the this files.
Backup = image = archived copy of selected partition(s) (usually several partitions are needed to boot Windows)
If the boot drive is damaged, it will not be possible to boot from it and run the HDM for recovery, therefore Recovery Media must be created in advance.
IMHO the operation you planned to increase the system volume to the entire disk is safe, does not threaten anything.