Sometimes the camera and program are working great, and then all of a sudden, every capture fails.This usually is caused by a disk failure:
Check if the hard drive is full. You can fill it up quickly shooting RAW files!
Check if someone moved or renamed the folders out from under you.
For an external or network drive, check if it was removed or disconnected.
Check if the permissions for the folder/drive are not allowing you to write to it.