Printing is evil. Less evil but related is scanning. I don’t have to scan documents very often, but I needed to a few weeks ago. I fired up XSane and it immediately quit because it couldn’t find any scanners. I have an all-in-one, and the printing worked, so I felt like it should probably be able to find the scanner. Running `lsusb` showed the device.
“Maybe it’s a problem with XSane”, I said to myself. I tried the `scanimage` command. Still no luck. Then I ran scanimage as root. Suddenly, I could scan. My device was on /dev/bus/usb/001/008, so I took a look at the permissions. Apparently the device was owned by root:lp, and the permissions were 664. I wasn’t in the lp group, so I couldn’t use the scanner. So I added myself to the group.
I’m assuming something changed when I upgraded to Fedora 21, since it had worked previously. Perhaps a udev rule is different? In any case, if you happen upon the same problem, enjoy this shortcut answer so that you don’t spend an hour trying to debug it.