what exactly is the error message on Windows while trying to access usb1 share
EDIT
on the 2nd Pi, did you tried to mount the USB1 share manually?
sudo mount -t cifs -o username=dietpi,password=<share_password> //SHARE_IP/usb1 /mnt/samba
what exactly is the error message on Windows while trying to access usb1 share
EDIT
on the 2nd Pi, did you tried to mount the USB1 share manually?
sudo mount -t cifs -o username=dietpi,password=<share_password> //SHARE_IP/usb1 /mnt/samba