A. Disk errors generally mean that your disk has a bad sector, and the computer cannot access or save your file.
Bad sectors are portions of the disk that can no longer save information.
When this happens, stop using the disk immediately. Usually each badsector on a disk will remove a few lines of an MS Word document, for instance. When you buy disks, remove the individual plastic wrapper they are shipped in. Putting them in and out of that envelope can build up static electricity and damage your disk. You should keep disks in a container, however.