They always warn you about upgrading to the first release. “Let them fix the bugs in the .1 before you upgrade,” they say.
I never listen.
In Sierra, I’ve run into an interesting problem where the first time I mount my Time Machine USB backup drive post-launch, it mounts and works just fine. I can then tell the OS to eject the disk, which it seems to. Except, in Disk Utility, it still thinks it’s there. And now this phantom disk prevents it from ever being mounted again, until I reboot.
I’ve tried command line mojo (diskutil and hdiutil), and both will say they’ve unmounted the drive, but it still shows up. Eventually, diskutil stops responding.
Reading online, there’s lots of folks reporting similar issues, but no one seems to have found a particularly good solution, other than “reboot.”
We’ll see if it gets fixed up in the .1 release. Until then, off to reboot.