Trying to format WD Ext HD Essential to Mac, get this message: "Cannot dismount volumes" NOW WHAT?

Just purchased another WD Ext. HD and trying to do the Quick Format option but it tells me it can’t dismount volumes as one is busy…what does that mean and how do I fix it??? Please help…and THANKYOU!

If I were in your shoes, I would reboot the computer and unplug the drive in question while the system is off.  Whenever a Mac does something stupid or unexpected, which is far to often for my tastes, I reboot it.  Plug the drive back in and try again.