Backup Program not working

Trying to use the backup and installed the program as instructed, but when I go to backup I get the message “The destination may not be writeable Would you like to continue?” This is both with a thumb drive or Mini SD card in my Nvidia Shield.

I completely performed a fresh install and setup the add-on’s how I want them. So I wanted to make a copy and restore it to a second Shield, but as I said, every time I try I receive that above error message. I did go to settings in backup and made sure to select the device I wanted to save to, but still that error.

I greatly appreciate your help in resolving this issue.