1. What Operating System?
2. Can you use File Explorer (Windows) or Finer (MAC) to see the hard drive and the files on your computer?
3. Are you sure you were looking at the correct drive.
4. And when you accomplish task (2) double check the hard drivers designated letter, in case it changed (due to conflict). Ilightroom thinking it is looking at an SD card as opposed to a hard drive makes me ask (3) and (4).
Try restarting the computer.