Hi, @lostados 
Let me start by saying that I don't use "Déjà Dup Backups" (the native Ubuntu MATE application that is available with the name "Backups" in the Ubuntu MATE "Accessories" menu). So, I was hoping that someone that does use that backup tool would reply to you. Since no one else has replied yet, I'll try to give some help.
Wen you say "WD", I'm guessing you're referring to a "Western Digital" hard drive, right? And what does "PL" stand for in "WD PL"? Is that an external USB hard drive that you're connecting to your home computer so you can back up the content of your internal hard drive to that external USB hard drive? What's the type of the filesystem that external hard drive use (ext4? FAT32? NTFS? other?). Can you see if the external USB hard drive is getting properly mounted by running the following 2 commands in a "Terminal" window, and see if they return any output:
lsblk | grep -i '/media'
mount | grep -i '/media'