If you're using the stock file manager, it probably isn't up to the job. Go to the Play Store and download Root File Manager (a yellow icon) and install that.
You don't need to root the device to use the utility. Open it and tap Storage then extSdCard and you should see the files are safe and intact. Tap DCIM/ if you want to check on your photographs, as an example.
It's a good practice to take the micro card out occasionally, buy a converter carrier the size of a full SdCard and connect it to your PC to backup the data.