MediaBank HS-Ri (Hot Swap Raid Internal) is

Featuring two easily removable hard drive trays and hardware support for
Raid 1 (mirroring), HS-Ri is one of the most secure storage solution
available today. Data is written to two hard drives simultaneously,
preventing total data loss if a drive were to fail.
Hardware support for mirroring removes potential software conflicts and
guarantees a faultless and automatic backup of your vital data.

MediaBank HS-Ri ships with a spare hard drive tray, allowing you to keep a
drive containing all your data in a secure location! Simply swap a drive
and you will have performed a backup!

link: