FWIW, I personally would not use any disk drive that is internal
to a Mini for a backup of anything. The reason for that is if something
should happen to the Mini, your backup will be unavailable. And of
course, that is when you will need the backup, Murphy's Law. getting a
drive out of a Mini is no small task.
Personally, I keep any backups on external drives, as they can be
easily moved and replaced.
The only case where I would keep a backup internal to a computer
would be in the case of a Mac Pro where you can easily remove and
swap hard drives.
Just something to consider.
Oh okay, so you have the server model. Just making sure as you can purchase and install the Server app separately on any Mac.
Well, then by default the disks are setup as RAID 1, means that the second disk is an exact mirror of the first. So, then you have a backup already. Unless you changed the RAID setup, of course.
If you don't run RAID 1, then I agree with woodmeister: get an external drive and use Time Machine or another app that you trust.