Backup target is "offline" and won't backup

I have an 8TB MyBook as a backup for my computer and another 5TB external drive with all my photos and videos. The 8TB recently stopped backing up and indicated it was full and “offline”, but it can’t be full because I have nowhere near 8TB of data on my computer and photo drive combined. So I ran the checks in the drive utilities app - the smart check was ok, the quick test was ok but the complete test failed. So I formatted the drive and tried to complete test it - failed. It still says it’s offline, even though I can hear it humming away and the computer is recognising it. I copy/pasted a file to a test folder and it worked. How do I get it to not be offline?


Please refer to the following KB article: