Once you have done a successful back up of your site collection you will try to restore the site collection using the following command.
stsadm.exe -o restore -url http://server_name/sites/sitename -filename “c:\\backup.dat” -overwrite
Unfortunately you may get this error.
The site collection could not be restored. If this problem persists, please make sure the content databases are available and have sufficient free space.
This issue occurs if the computer that is running Windows SharePoint Services has insufficient free disk space that is available to host the restored Windows SharePoint Services Web site. This issue may also occur if the Windows SharePoint Services database has reached its maximum allowed size.
Find the Microsoft support article here kb926061