In Use TurnKey to Simplify ownCloud Installation, you may host your TurnKey ownCloud. In Access Shared Folder on VirtualBox from Guest Linux VM, you may use a shared folder on host from guest VM.
How about host TurnKey ownCloud and ask it to save all backup files in a shared folder on host to save efforts increasing the Virtual Machine Disk? Even on a different internal hard drive or external RAID for better protection?
You need to manually change the ownCloud configuration file and add www-data to vboxsf group in Linux.
Configure ownCloud to Save to a Different Directory
ownCloud save all files in /var/www/owncloud/data. You may modify config.php which is in /var/www/ownclou/config directory.
Look for ‘datadirectory’ and replace ‘/var/www/owncloud/data’ with your shared folder full path.
Add www-data to Group vboxsf
Next, add www-data to vboxsf group user by edit the vboxsf:x:1001: in /etc/group:
Reboot your VM and it should save files to your shared folder now.