If you own an old or high-end digital camera you are used to compact flash cards. The chromebook has a card reader for SD-cards only. As the chromeOS is a relativly new OS I was not sure if all the drivers are in to run external card readers. So, I bought a very recent model (with USB3.0) and tried it out myself.
I used the USB 3.0 SuperSpeed Multi Card Reader (there are many colors available). Plugged in to the USB2.0 port (Acer C720 right side) it worked perfectly, but in the USB3.0 port (Acer C720 left side) my chromebook crashed sometimes. I reported that issue to google and hope they will fix it soon.
I was able to copy my raw picture files from the compact flash card to the gdrive using my Acer C720 and the Hama card reader (as auto backup is not supported for raw yet). Having them in the gdrive allows further photo editing by google+ and the photographic workflow is complete even without the auto backup function. The card reader works as expected by plug-and-play without additional driver software.
You can support my product testing by using the links below to buy your card reader.