Older Pro Edition not mounting to MacBook Pro

I have a 4 1/2-year-old Pro Edition that I bought at the same time as my 17" MacBook Pro. External has always worked fine. I don’t keep the Pro Edition connected to the MacBook all the time. Yesterday, I connected it with the MacBook off, as I do every time. When I started the MacBook, the Pro Edition acquired power, but wouldn’t mount to desktop. Just a single, steady light. This has never happened previously. I disconnected the Pro Edition from the MacBook after turning it off, but Pro Edition continues to run with steady light. I don’t want to disconnect the power to the Pro Edition while it is running. Any suggestions or insight would be appreciated.

Hi dude, do you see the drive on disk utility? (Go> utilities> disk utility)