Storage maths: how much do you actually need?
Almost everyone buys too little redundancy and too much capacity. The two mistakes come from the same misunderstanding.
The rule: RAID is uptime, not safety. It protects against a drive dying. It does nothing about deletion, ransomware, fire, theft or you typing the wrong command — which is what actually destroys data.
Raw vs usable
Two 8TB drives mirrored give you 8TB usable, not 16TB. Four 8TB drives in single-parity give you 24TB usable. Then the filesystem takes its cut, and you should not run a pool past about 80% full because performance falls off a cliff. Plan on roughly two-thirds of the number on the box.
| Config | Raw | Realistic usable | Survives |
|---|---|---|---|
| 2 × 8TB mirror | 16TB | ~6.5TB | one drive failing |
| 4 × 8TB, single parity | 32TB | ~19TB | one drive failing |
| 4 × 8TB, double parity | 32TB | ~12.5TB | two drives failing |
Why CMR matters more than capacity
Shingled (SMR) drives write by overlapping tracks, which makes them cheap and makes rebuilds glacial — days, not hours, on a full array. A rebuild is precisely when your array is least protected, and stretching that window out is how a single failure becomes two. Buy CMR NAS drives (Amazon search, opens in a new tab) and check the model, because manufacturers have shipped SMR without labelling it.
Buy from different batches or vendors where you can. Drives from one batch tend to fail near each other, which is exactly the correlation your redundancy assumes away.
3-2-1, applied honestly
Three copies, on two kinds of media, one offsite. In a real homelab that is: the live copy on your NAS, a second copy on a different machine or an external drive (Amazon search, opens in a new tab), and one copy that is not in your building — cloud, or a drive at a relative's house that you swap every few months.
Most homelabs have the first two and stop. The offsite copy is the one that matters, and it is the one that requires a habit rather than a purchase. If you know you will not maintain a rotation, buy cloud backup and be honest about it — an unrotated drive in a drawer is a copy of last February.
How much to buy
Take your current data, double it, and buy for that. Not four times. Drive prices per terabyte fall steadily, so capacity bought early is capacity bought expensive, and an empty bay costs nothing to leave empty.
The exception is bays, not drives: buying a four-bay enclosure (Amazon search, opens in a new tab) and populating two is the cheapest possible upgrade path, because adding a drive later is easy and replacing an enclosure later is not.
The picks
Starting out: four-bay enclosure, two CMR drives mirrored, one external for offsite. Expand into the empty bays when you are past two-thirds full.
Media libraries above ~20TB: double parity. Rebuild times at that size make single parity uncomfortably thin.
Photos and documents only: a mirror plus real cloud backup. You do not need an array; you need the offsite copy, and versioned cloud storage covers the failure mode that actually gets people.