I now have a 2nd EX2 Ultra that I use for backups. How do I remove old backup files from my other EX2 Ultra before I use it to store music?
It may depend on how and what program you were using to backup what ever you were backing up. One can always just perform a format of the hard drive to erase all user data. Otherwise one can use Windows File Explorer or Mac Finder to access the My Cloud connected to the same local network and delete any files/subfolders they desire to delete. One can usually delete entire Shares through the My Cloud Dashboard.
See the My Cloud Dashboard’s embedded Help for more information on how to format the My Cloud drive (multi bay My Clouds) or delete Shares. Or see the WD Support site (link at the top of page) for your device were you can find User Manuals and additional WD Support Knowledgebase Articles.
Because you are using an EX2 Ultra (two of them), you may want to see the dedicated subforum for the OS5 firmware EX Series of devices (assuming your EX2 Ultra is running OS5) where it appears most of the EX2 Ultra discussions may be taking place.
My Cloud EX Series