RAID is the reason a network enclosure can lose a drive without losing data, and it is also the feature whose limitations are most consistently misunderstood. Choosing a level is a trade between usable capacity and how many simultaneous failures the array can absorb, and neither of those is the same as being protected against losing the data.
What each level gives up and survives
| Layout | Minimum drives | Usable capacity | Survives |
|---|---|---|---|
| Striping | 2 | All of it | Nothing |
| Mirroring | 2 | Half | One drive failure |
| Single parity | 3 | All but one drive | One drive failure |
| Double parity | 4 | All but two drives | Two simultaneous failures |
| Mirroring with striping | 4 | Half | One drive per mirrored pair |
For two-bay enclosures the choice is between halving capacity and accepting no protection at all, which is not a real choice for data that matters. For four bays and above the interesting comparison is between single parity, which is efficient but intolerant of a second failure, and double parity, which gives up another drive's worth of capacity in exchange for surviving one.
Rebuild exposure, the risk that is not on the datasheet
An array in a degraded state has no tolerance left. Rebuild time rises with drive capacity, so replacing a large failed drive in a single-parity array can occupy the array for a day or more, at reduced performance, with every remaining drive working continuously. If a second drive fails during that window, a single-parity array loses everything in it.
That window is why double parity is usually the right answer for arrays built from large drives, despite costing an additional drive's capacity. The larger the drives, the longer the vulnerable period, and the more the second parity block is worth having.
Redundancy is not backup
RAID protects against one thing: a drive failing. It does not protect against deletion, ransomware, a fault in the enclosure's controller, fire, theft or electrical damage, because every one of those events reaches all members of the array at once and in the same way. An array is one copy of the data, however many drives it contains.
The arrangement that is actually safe is three copies on two different kinds of media with one held elsewhere. For a household, that means the array plus a second copy on separate hardware plus at least one copy that is not in the same building. Snapshots protect against accidental changes, but they live on the same volume and do not replace that off-site copy.
Where to start
- NAS buying guide — bays, drives, tolerance and running cost taken in order.
- Does a household actually need a NAS? — whether the enclosure is the right answer at all.
- DIY vs. prebuilt NAS — how the same layouts are configured on each platform.
Where to buy a home NAS RAID configuration
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