Card Reader not recognized?

As you can see from this screenshot, my card reader (with an SD card from a Canon camera shows up quite well on my desktop running 10.8.3:

Have you checked that you have all the options checked in Finder Preferences > General (that will include any card reader as it is perceived as an external disk):

Hello BananaRepublic,

Welcome to the Apple Support Communities. In Lion 10.7 and in Mountain Lion 10.8, the card readers show up in Finder on the side bar. They do not show up on the desktop like in previous versions of the operating system. The following article will walk you through the changes

Mac Basics: The Finder

http://support.apple.com/kb/ht2470

8. Devices - A device connected to your computer, such as a DVD, USB device, or your Time Machine backup disk.

Regards,

-Norm G.

With Finder active (top menu bar), click on Preferences.

Have you checked the reader in another computer? Does it show up there? Or have you recently switched to another SD card (if it is a new high speed card, the reader may not be able to handle it).

Shot-gun answer follows, and all this presuming that you've verified the sidebar stuff discussed earlier...

Disconnect all external devices — hubs, disks, whatever — except the card reader, and try hot-plugging it again. USB devices can fail, or can draw too much power, and one wonky device can drag down the whole USB bus.

If you have any add-on tools that chat to devices on the USB, such as USB serial dongles, see if that software is current, and also look at removing it. One of those could conceivably be getting tangled with or confused by these USB devices.

On no evidence, I'd also try a safe boot, try the USB Card Reader, and then shut down; see if the behavior with this device changes,

Also try the SMC reset, as that can reset the state of the device.

Also see if other USB devices work and see if other USB ports work, as it's conceivable you've lost the USB port(s) on the iMac.

If you have access to the developer tools and want to poke around at a lower-level than usual, there's a USB Prober tool that's part of the Xcode hardware tools package.