It's not a limitation, it's a requirement, as equal and valid a requirement as two drives is for RAID-1, or three drives for RAID-5. 3-way mirroring doesn't even make any sense without 5 drives.

A mirror only lets you use one disk in a stack in usable space (50%, or less if for some reason you want a double redundant mirror, i.e. three disks). A RAID-Z depends on the amount of parity ...

WD’s G-RAID Mirror is an external enclosure housing two 3.5-inch hard drives. It provides up to 48TB of storage with the drives in striped (RAID 0) mode, or half that in mirror mode RAID 1.

Hardware-level RAID requires a dedicated RAID controller card to offload all the heavy lifting from the CPU, enabling faster performance. It also generally has a lot more features to enable redundancy ...

Most desktop PCs will have RAID support, but it's not a guarantee. Storage Spaces is more easily accessible, as long as you have the number of drives you need. They can even be external drives .

You have your choice of 10 different models when buying the Guardian MAXimus RAID 1 Mirror. It's compatible with Apple OS 8.6 – 9.2.2 and MAC OS X 10.0 and up. Pricing starts at $230.

That’s a lot of space for stuff, either way. The Gemini is stylishly rendered in black metal ... Almost 100MBps quicker than the SanDisk G-RAID Mirror using the same 24GB WD UltraStar DC HC580 HDDs.