RAID-5 is fault-tolerant meaning it can survive the failure of one of the drives: swap it for the spare and keep working—no downtime (keep a cold spare on hand, so buy one with the device, e.g. a 5th drive for a 4-drive unit).
MPG regularly receives emails on whether RAID-5 or RAID-4 can be partitioned into multiple volumes (or how to set up a Thunderbay 4 unit): a Thunderbay 4 can be used as a single huge RAID-5 volume, or (recommended approach) partition it into three separate RAID-5 volumes.
For example, instead of one 15TB volume, make three 5TB volumes. Or not use RAID at all, and use the drives as single non-RAID drives if fault tolerance is not needed (use a disciplined backup strategy). The MPG recommendation when using RAID-5 for most users is to NOT use one large 12/15/18TB volume, but to “chunk” it (partition it) into 4/5/6TB volumes that thus match the 4/5/6TB capacity of single-drive external backup volumes. This has the operational advantage of simplicity when making backup clones.
See the MPG review of the OWC Thunderbay 4. MPG has five of these units—favorite storage device ever.
See aso Using Cloning as a Backup Strategy.