I would try different hardware ... different cable, different USB port, even trying it on a different computer.
Are you seeing any error message?
If no then go to your Lightroom preferences > General tab> at the bottom of dialog box Prompts> select "Reset all warning dialogs".
Try another import. Any error message now?
Hint! Check the Drive / Folder / sub-folder where you are copying the file to, and ensure you have full Read and Write permissions. Your update to Yosomite may have changes things.
I am having the same issue too. I just photographed a wedding and when I was importing it took 6 hrs to import 12 images. It just gets frozen/stuck and nothing happens. I don't have any other software (like photoshop) to be able to import my RAW files to DNG. Have you found a solution yet to the problem? I was a second shooter for this wedding and the main photographer is having the same issue trying to import the images from my cards to her Lightroom and she has a windows computer not a mac. So I don't think it's just a mac issue. We think it's a Lightroom issue and they just aren't supporting SD cards.
I really hope that you (or anyone else) has found a solution and can help.
**The 'error message' that my friend got said something along the lines of the card not being formatted correctly which I know is bogus bc I format all my cards before each shoot and I've used them before on other shoots with no problem.**
when I was importing it took 6 hrs to import 12 images
Bad card reader, or bad card, is the most likely cause
The fact that it worked properly a week ago is irrelevant.
How do you know it's not the card, if both people have the same issue? Do you have other cards? Can you import from them without problems?
We think it's a Lightroom issue and they just aren't supporting SD cards.
SD cards have been supported in Lightroom since the mid-1800s.
Of the 3 cards 2 aren't working. This string started off with person having the same issue. Recent update making import from so cards pretty much impossible