Did the move to El Capitan a couple of days ago. Seems pretty painless thus far. One glitch. The Seagate external drive that I was using for Time Machine is no longer recognized. I've plugged/unplugged, restarted, but EC does not acknowledge that drive. The light on the Seagate comes on when plugged into the Mini's USB port and the drive is spinning. But that's the extent.
The Toshiba drive being used for Super Duper pops up and all is normal. Is there something I can do to recover the Seagate? It doesn't show up as an option under "select disk" in the TM prefs. Did the Seagate just coincidentally expire at the same time the OS update was happening?