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Silent data corruption is common

You just don’t know it. Many low-end RAID controllers don’t report problems, figuring you’ll never notice. If you do notice, months later, what is the chance that you’ll know it was the controller’s fault?

Back up is better than insurance

Insurance is designed to protect you against damaging but uncommon events. But data loss is very common. Backup isn’t insurance. It is simple digital hygiene. You’ll use it again and again.

What are disks made of?

Hard drives sit at the bottom of a stack of hardware and software that usually gets your data from your CPU to the disk and back. But there are a lot of places where things can go wrong.

Here’s a partial list:

Media: those beautifully plated silver disks are subject to a couple of major problems:

* Flipped bits: when a read-only track sits next to frequently written track, the extraneous magnetic field from the writes weakens the magnetization of the read-only bits until your disk can’t read it. Normally disk ECC corrects these errors, but not always.

This is why disk fanatics periodically zero-out their disks and reload all their data. I’m not recommending this, just noting the practice.

* Physical problems, like a piece of dust, can scratch the disk and/or create enough heat so the head stops reading momentarily. Depending on severity the disk may remove that block from use or begin a death spiral into oblivion.

Wear out: disks have a lot of moving parts. In a 7200 RPM drive the disks are spinning 120 times per second compared to the 500 RPM of a CD drive. After a few years the motor can start to go. It may become slightly erratic, so some bits get squeezed and others get smeared.

The arm that moves the heads may can move dozens of times per second. When the bearings get loose it can go off track and corrupt data on adjacent tracks.

Electrical: if the drive power supply fails your drive will shut down. But if it is slowly degrading it can create extra heat or power surges that affect already marginal components. Component failures leading to sudden death are not seen by SMART reporting, which is one reason why SMART isn’t much use.