Roomba 500 Series Error 2? Clean the brush gearbox.
The Roomba kept telling me I needed to clean the brushes after about 30 seconds of cleaning. "Error 2. Open Roomba’s brush cage and clean brushes." Sigh.
It was also making a sick click-click noise as it tottered around. Visiting http://homesupport.irobot.com/app/answers/detail/a_id/740/p/788 suggested taking out the brushes and turning it on, to see if you still here the click click noise (25 seconds into the video).
After opening a report with iRobot support, and finding out that it needed a $50 replacement part, I decided to just take a look at the assembly. After removing the bottom cover and then the brush cage assembly, I cleaned out the gear compartment which was completely clogged. Put it all back together and it's now working like new. Success!