SD card not showing any more - CNET Mac OS X Forums

In that case, I assume you have tried rebooting the computer, but do that first if you haven't. Then if still nothing, open up disk utility and put a card in the reader. Give it about 30 seconds and see if anything new shows up in the list. If it does, then my guess would be for whatever reason the cards aren't mounting, and if you hit the mount button it will become accessible. If it doesn't, then you probably have a bad SD card reader, maybe a bad cable. Unlike Windows, my experience has always been that OS X I would expect the SD card slot on your monitor gave out, but it's quick and easy to check your USB uplink cable to the monitor. You could even replace it, and only be out a few bucks if it doesn't solve the problem.

In my experience, OS X is kind of picky about what card readers it will recognize, so you probably got lucky with the Dell monitor, and the fact that other card readers didn't work doesn't necessarily say anything... Unless you know for a fact they worked earlier. I will definitely give you points for thinking along the right general lines however.