Comment 7 for bug 910964

We believe this to be a gphoto2 problem. If you could do the following, that would help verify this (with your camera attached):

$ sudo apt-get install gphoto2
$ gphoto2 --shell

The gphoto shell is like a command-line for your camera and attempts to lock it before using it. (You'll need to unmount it from Nautilus before beginning.) If you get the same error Shotwell is reporting, then it's a gphoto2 bug -- there's not much we can do on our end.