I have two 4TB USB drives that I’ve been using for backups of my WDMycloud OS5. I reformatted one of them recently and the MyCloud no longer recognizes it. I plug it in, but nothing shows up on little USB device widget at the top. I’ve tried rebooting the MyCloud with and without the USB drive attached, and it still doesn’t recognize it. I formatted the drive as NTFS from my Windows 10 PC.
I’ve found a number of threads about this problem, most of which are several years old, but could not find anything that worked for me.