MacMini Server???

Apple's software RAID implementation is terrible anyway, prone to having failed status but not notifying anyone that it's failed until someone happens to open Disk Utility. Performance is also unremarkable at best.
One of my servers uses a Promise Pegasus2. That works much better than Apple's RAID ever did.

 

The mini was never suitable for critical server use anyway, which requires more redundancy than just drives. Mine does just fine with regular backups -- never lost any data. When I had a server model (the original one, 2009) I kept the two drives independent.

 

You've got the WD MyBook Pro or WD MyBook Duo/ThunderBolt Duo which work pretty well too.

I agree with 'talmy' that redundancy requires much more than RAID. If its 'mission critical' a Mac Mini probably isn't the best idea for you. As an idea we have multiple servers running any important workloads with regular data replication in place which seems to work well for us.

 

If you really need mission critical 24/7 Mini, you might consider a bootable thunderbolt RAID 5 array. Use the internal HD as a backup for the OS/boot volume, or other non-essential storage, backups, etc.

The other thing to consider, if buying a new Mini, is that internal SSD storage option for a boot volume is fairly robust. Should be more reliable than any HD, and perhaps a HD mirror.

We have a setup such as this at work with an TB RAID 5 array, and it is great. Not a 24/7 requirement....if the SSD failed, I could still boot to the OS backup/Clone from an external drive in a pinch...until repair/replacement was ready.

 

I'm with you. I only recently found this out do to a lightning strike taking out everything connected to the rear wall. UPS and lightning protection circuitry both burned through. Fortunately, insurance paid for everything, but my MacMiniServer was not available. I'm currently running a MacMini with Server software.