I had the same problem with two readers. I went to the Apple store and found that the problem was neither with the readers or the SD cards. Actually, Apple changed the software for the readers. When you plug it in, the name of the card or camera will appear. Click on the name of the card or camera under utilities, and it should work.
I have the software update. When I plugged the reader in, the name of my camera came up as it usually does. When I selected the pictures and pressed transfer, the pictures disappeared. Thanks anyway for taking the time to help. I appreciate it.