Redo Backup of Linux Mint 19.3 (Cinnamon) to Toshiba external USB HDD

I recently installed Linux Mint 19.3 (Cinnamon) on my Samsung laptop. All went good. I then rebooted my laptop and backed up my newly installed OS, via Redo Backup, to my external USB Toshiba HDD, which I had previously formatted to EXT4.

All "appeared" to go well; however, when I rebooted my laptop, I got a grub error and what looked like a DOS prompt. My newly installed OS would not boot! So, begrudgingly, I had to endure a second OS installation!

Any idea what happened, because I'm getting ready to try to do the Redo Backup again. How should my Toshiba actually be formatted?

Thanks, guys!