By default, Mac minis with RAID arrays are set to RAID1. That means the pair of drives are mirrored for availability.
If you’d like a different RAID configuration at the time of order, open a support ticket with your request after creating a new subscription.
Our support staff can also handle custom requests. One popular, virtualization-focused option on the Mac mini with quad core i7 CPU is removing the raid array and replacing one of the disks with an SSD. The user can then install VMware ESXi on the SSD with a few high-performing (or all) virtual machines while using the remaining HDD for low-performance virtual machines and/or backup.
An important disclaimer: RAID in any form is not a backup. RAID provides availability or speed based on the format. If you lose a disk in a RAID1 array, you have another disk available to rebuild the RAID array after adding a new disk. Other forms of RAID offer lesser or greater amounts of availability by way of parity. The standard RAID levels page on Wikipedia is a great resource for learning more.