I can't see usb-drives and archives in the sidebar of the finder anymore When I plugin a usb-drive or open dmg-archives they don't appear under the 'Devices' section in the sidebar of the Finder anymore.

The drives are there when I check with 'Disk Utility', they appear on the Desktop and I can browse the files if I go to /Volumes.

I have 'Hard disks' and 'External disks' checked in the Finder preferences -> Sidebar tab.

Any ideas on how to get them to show up in the sidebar?

I'm using MacOS Lion 10.7.5.
In your Finder sidebar, can you see your internal hard drive listed under Devices? If you hover over Devices you should see either Show or Hide just to the right of it, perhaps it's set to hidden?