Can't do anything to hard drive because it's "in use" but it isn't

I just got the Western Digital Elements 2TB hard drive today, and I'm trying to Erase it so I can use it on my Mac in the format of Mac OS Extended (Journaled). However it says:

"The volume 'Elements' on disk7s1 couldn't be unmounted because it is in use by process 19754 (diskimagesiod)

Couldn't unmount disk : (-69888)"

I tried looking up "process 19754 (diskimagesiod)" but there literally wasn't a single result. I'm so confused what is up with my computer... I've been having issues mounting a .dmg file (have tried basically every single thing I could find suggested on the internet but it just won't mount) and I wonder if this is related. Within the WD Discovery app, it shows the hard drive as Read Only and says to write to it, I have to reformat it to HFS+, so I know the format I was trying was correct. What does this error message mean?

When I clicked "Unmount" it says "Failed to unmount 'Elements' because it is currently in use." I don't know how it's in use, the WD Discovery is closed and I don't see how else I could be opening it. Does it count to just have it plugged in...?

Edit: Now I'm just trying to eject it, and it said one or more programs is using it even though I had absolutely nothing running except the Finder which can't be force quit out of.