You should search this forum for "fssnap" and "ufsdump" and/or "fssnap ufsdump"
Here is a thread where I describe the process:
Backup root disks
If you have more than one filesystem then you should backup each one separately to give you greater options for recovery if a disk fails.
You can send a ufsdump to a tape (which is typical), over a NFS link to another system, to a SAN LUN, or any other storage you can address.
fssnap freezes the filesystem (sending the ongoing changes to a journal) so that the filesystem is not moving whilst being backed up. Remember to release the snapshot which then allows the journal to update the filesystem and bring it up to date.