Before this:Picking a board
Storage & SD cards
Key takeaways Storage is the most common failure point of an SBC build. SD cards are slow and wear out under constant writing; buy a name-brand A2-class card — or better, a “high endurance” card built for continuous recording. The real upgrade is booting from an SSD (USB or NVMe): faster in every way and dramatically more durable. Whatever you choose, capacity is cheap — buy bigger than you need — and assume the medium will eventually fail: the defence is the backup image, not the perfect card.
The board you picked last lesson stores its entire life — OS, configuration, recordings, logs — on one small piece of flash. This lesson is about choosing that piece well, because a cheap card is the single most likely thing to take your appliance down.
Why is storage the weak link?
Flash memory wears: each cell survives a limited number of write cycles, and an always-on Linux system writes constantly — logs, databases, temporary files, and (for a scanner) a steady stream of audio recordings. A quality SSD manages this with sophisticated wear-levelling across abundant spare cells. A bargain SD card has the cheapest possible controller and no spare capacity to speak of. The typical death is not dramatic: the system gets slow, then read-only, then unbootable — often months in, long after you stopped watching. SD-card wear in Unit 5 covers reducing the write load; this lesson is about buying hardware that can take it.
How do you read an SD card label?
SD cards carry a fistful of overlapping ratings. The ones that matter:
| Marking | Means | Why you care |
|---|---|---|
| A1 / A2 | “Application class” — random read/write performance | The best predictor of how fast Linux feels; buy A2 |
| U1 / U3, V30… | Sustained sequential write speed | Matters for video recording; less predictive for OS use |
| “High endurance” | Built (with wear headroom) for continuous recording, e.g. dashcams | The right product for a 24/7 appliance |
| Capacity | 32/64/128 GB… | Bigger cards also last longer — more cells to spread writes over |
Two buying rules beat all the ratings. Buy a name brand from a reputable seller — counterfeit cards (fake capacity, dismal speed) are rampant on marketplaces. And bigger is better even if you don’t need the space, because wear-levelling across more cells means each cell is written less often.
Rule of thumb: for an always-on appliance, a 64 GB+ name-brand high endurance card is the floor. The few extra dollars are the cheapest reliability you will ever buy.
When should you step up to an SSD?
An SSD — attached over USB 3, or on boards that support it, NVMe (a direct, faster connection) — changes the storage story entirely:
- Endurance: real controllers, real spare area, wear ratings measured in written terabytes. Effectively removes the wear worry.
- Speed: boots, package updates, and database work go from sluggish to snappy; random I/O is often 10× an SD card’s.
- Reliability: failure modes are rarer and more graceful.
The costs are modest: a bit more money, a bit more power draw (budget for it — next lesson), and slightly more setup, since you configure the board to boot from USB/NVMe. For a build whose whole point is running unattended for months while writing recordings continuously, an SSD is the sensible default when the board supports it; the high-endurance SD card is the budget path that also works.
How much capacity does a scanner appliance need?
Sketch the arithmetic rather than guessing. The OS and GopherTrunk fit in well under 10 GB. Recordings dominate: compressed per-call audio is small individually, but a busy system recording around the clock accumulates gigabytes per month, and call-log databases grow alongside. A retention policy (delete recordings older than N days — the Scanning module discusses this) caps the total. With 64–128 GB and a sane retention policy you will run out of interest before space. Log growth is the other quiet consumer — SD-card wear shows how to keep journals bounded.
What’s the mindset — durable media or disposable media?
Here is the reframe that separates frustrated beginners from calm operators: assume the storage will fail eventually, and make that boring. You’ll build this properly in Backups & images: keep an image of your configured system, keep configs exported, and a dead card becomes a ten-minute re-flash instead of a lost weekend. Buy quality media to make failure rare; keep backups to make it irrelevant. Do both.
Quick check: which SD card is the best fit for a 24/7 recording appliance?
Recap
- Storage is the most common SBC failure point: flash cells wear out under an always-on system’s constant writes.
- Read card labels for A2 (random I/O) and high endurance (continuous-write headroom); buy name-brand and bigger than needed.
- An SSD over USB 3 or NVMe is the real upgrade — faster and dramatically more durable; the sensible default for an appliance when the board supports it.
- Size capacity by arithmetic: OS is small, recordings plus retention policy decide the number.
- Mindset: quality media makes failure rare; the backup image makes it irrelevant. Do both.
Next up: Power supplies.