> All the other backups are running fine I don't understand why it is saying unable to open device after backing up 2.6TB. I would really appreciate any ideas.

Back when I began building computers, problem with the motherboards, chipset, or ram weren’t always apparent. One thing I discovered was that copying a very large amount of files over the network to the machine being tested was an excellent way to know if there were any hardware problems.

You might do a simple test of your own - copy a couple of terabytes worth of files by hand to the pc you're using as backup storage. It should copy flawlessly.