You can use floppy disks. These are obviously no good for taking a full backup of a 2GB disk, but they are OK for making backup copies of the files you are currently working on. You can use a writable CD drive to make your own CDs. These are fairly expensive at the moment but expect the price to fall. Unix and PC files do not have version numbers, so in general you can't go back to the original version.
Computer viruses can cause loss of data.
If your Sable or Ermine account is over quota with no grace period, and you use pico to edit a file, the file is zeroed and when you try to write out the edited file, it fails. Both the original and the edited versions are lost. Make copies of important files, even if this means FTP'ing them to your workstation. Make a copy of a file before editing it, or use an editor which automatically takes a backup copy. Set protection on Unix files so that you can't delete them by accident.