Hi Robin,
Quite possibly the loss is due to a connection issue with your external drive. This doesn't necessarily mean a hardware failure; it might have been nothing more than your USB driver having a hiccup. It's generally not a good idea to save directly to any external media. You may be able to recover the file using data recovery software like Back2Life (
http://grandutils.com/Back2Life/). The important thing is to not make any changes (eg by saving new files) to the same media before running the data recovery software.