There is no reason for a backup to require a reboot unless you are planning to do something like clone the installation with Clonezilla
or a similar tool. I would advise doing more research before proceeding.
The simplest solution would be to use something like rsync to back up crucial files; figuring out rsync, though, is not necessary simple because the command offers many arguments and options; it took me several hours before I found the combination that worked for me. There is a GUI for rsync that has been well reviewed by Sourcetrunk
called Back in Time
Since Rivendell uses MySQL, you should investigate how to maintain, export, and backup MySQL databases. WordPress has a good introduction
A quick search for "backup rivendell" turned up this