If you're looking for a disk image option, there isn't one (other than pulling the drive out and using an imaging software as you mentioned).
Nokia does provide a way for you to backup the configuration and files and offload them from the appliance.
From Voyager, under Configuration - System Config - Backup and Restore. Specify a filename, select the things you want backed up (such as logs, home directories, currently running packages), and click Apply. You can even schedule backup jobs and transfer the backup via FTP/TFTP automatically, or do it manually whenever you need to.