how to format external hd after you use it in the time machine ?

installed 500gb external hard drive (model wd external media 5000bev) use in time machine on macbook pro mac osx system. dont want time machine anymore, want format use @ work. , system complains: not unmount disk . i've tried everything, not know else do. in advance .

 

what disabling time machine first and/or removing hdd physically via pulling cable , plugging again , trying format again?

 

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