I bought a new SD card so first I need to transfer the data from my old one (it came with my 3DS if that matters and it's a Toshiba 2GB) when I copy it onto my computer it says preparing to copy data and then it doesn't copy the data.
That sd card probibly has an error on it. I've seen it happen a few times on my 3ds's sd cards. Sadly it's often an error that windows can't fix. Did you remove that sd card while the 3ds was on, if so that can damage the data. So avoid doing that.
The last time I had it happen to me I had to remove digital downloads on that sd card using the 3ds's data menu, one at a time until the sd card could be read by my pc. I started by removing demo's since they don't tend to have save data i need to keep. It only took removing a few till I got to the demo that had the bad data in it. Once the bad file(demo) was removed I was able to copy what was left on the sd card to a folder, i created on my pc, and then Copy What was in that folder to the new sd card.