I’m in the same boat as the OP. My internal HD is near full (approaching what was once, at least, the recommended minimum free space of 10%). I simply wanted an external HD onto which I could transfer things such as audio and graphic files (the content of "My Pictures for example) and even install programs if I was so moved so as to increase (and preserve) the size of the available free space on my internal drive.
As I think I’m beginning to understand, with the WD software installed, this drive can only serve as the backup drive in support of that function. Have I got that right?
If so, I apparently need to uninstall the software and re-format this external hard drive. Either that or buy another for that use as well. Have I got that right?
On edit: I think I’ve answered my own question as I was able to “move” an entire directory from my “internal” to the “My Passport” partition of the external.