Herobook loses D: drive

It took 2 weeks to discover the hard drive has 2 partitions. Now that I have moved all docs, photos etc to the D: drive, it frequently disappears. When I finish with a doc and want to save it to the D drive (where I opened it from), it no longer exists. This is causing some very high anxiety and anger from frustration. WHY is this happening and how do I stop it?