we just came back from vacation 2 days ago; i checked time machine. it says latest back up is delayed; then latest backup failed; next backup when disk is connected. i have a WD external hard drive. i check to see that it is connected to my imac and is plugged in for electricity. restarted my imac--same problem. there is no lite on the WD; does it mean it died? or what can i try? is it less than 2 yrs old.