Seems a common problem but really wasn't able to bring up a solution for my problem. My Seagate Freeagent Go 250 GB wasn't even mounted when plugged in after the update. But did show up in disk utility so attempted an erase and reformat. 24 hrs later still in progress, obviously going nowhere. Attempted repair as well, nothing. Thought maybe the drive since about 3-4 years old and fairly close in size to my HDD used portion. Ordered a Toshiba 750 GB new and plug in. It mounted and I then tried to erase and format it. Maybe would have worked as is but again no go on the erase and format after 12 hrs plus.
What's the story here. Really want to get Time Machine up and running again. Apple care was useless, the girl "helping" me had such a bad case of laryngitis I could only understand every 3rd word. Ridiculous.