First we need to know if you connect to the system session of libvirt or the user. When running virt-manager as root i assume you didn't (manually) create a user session for root.
By default the system session stores its images in
/var/lib/libvirt/images/. This folder is owned by
You don't need to run virt-manager as root to connect to the libvirt system session!
Do you want to use the VM in a user or system (root) session?
If you want to use a user session you can create one by selecting
file → Add Connection …. Then select
QEMU/KVM user session as hypervisor.
No matter which session you can change the storage pool under
Edit → Connection Details. now select the storage tab. In the bottom left you can find a plus. Click this and add a new storage pool (storage location) and select some folder on your second drive. Or even the drive itself by changing the type from
dir to disk.
Make sure the directory belongs to your user (if user session) or the user that runs the process "libvirtd" this usually
If the folder has the wrong permissions (as indicated by your error, then change the permission via
chown or grafically using your file manager.