Layer 2

Mirror vs single-parity

Household language, not a RAID textbook. One class for two bays, one class for four.

2-bay mirror

Two drives, copies of each other. If one fails, the other still has the data. Usable space is roughly one drive. Simple, quiet, enough for a first photo library.

4-bay SHR / RAID 5-class

Four drives, one drive of parity. One drive can fail without losing the volume. Usable space is roughly three drives. This is the usual next step when the planning target no longer fits a pair of disks.

What redundancy is not

RAID is not a backup. Fire, theft, ransomware and a mistaken delete still win. If you told the planner you have no off-site copy, the result says so in plain language — and we show a category search for an external drive, not a pretend 3-2-1 appliance.