The Decay Problem

Long-term data retention in storage media is a real and often underestimated problem. Data stored on devices such as SD cards, USB flash drives, and SSDs can slowly degrade over time due to physical limitations of the underlying technology. Factors like age, temperature, humidity, manufacturing quality, and prior wear all influence how long data can be reliably preserved.

Excessive read/write cycles can wear out memory cells and reduce their ability to store new data. However, the opposite extreme leaving storage media completely unused for long periods can also be risky. Even without active use, stored data is not guaranteed to remain intact forever. Why does this happen?

Modern SD cards and other flash-based storage devices use NAND flash memory. In NAND flash, each memory cell stores data by trapping electrons inside a floating gate (or charge trap). The presence or absence of this trapped charge represents a binary value (1 or 0).

When data is written, electrons are forced into or removed from the floating gate using carefully controlled voltages. When data is read, the controller measures how easily current flows through the cell, which indirectly reveals how much charge is stored.

Over time, the trapped electrons can slowly leak away. As this happens, the voltage margin that distinguishes a 1 from a 0 becomes smaller, increasing the chance of bit errors. This gradual loss of stored charge is known as data retention loss, and it is the root cause of long-term data decay in flash memory.

Backups and Long-Term Storage

If you create a backup on an SD card and store it for several years without touching it, there is a real possibility that some of the data may degrade. Higher temperatures significantly accelerate this process, which is why flash media stored in warm environments tends to fail sooner.

Unlike DRAM, NAND flash does not refresh itself when read. Simply reading the data does not restore lost charge or repair weakened cells. In fact, excessive reading of old flash memory can sometimes increase the risk of errors due to a phenomenon known as read disturb.

What does help is periodically rewriting the data. When data is rewritten, the controller reprograms the cells with fresh charge levels, restoring proper voltage margins and allowing internal error-correction mechanisms to remap weak or failing blocks.

How to Maintain Rarely Used Backup Media

In many cases, backup media is kept only for archival purposes and accessed very infrequently. To reduce the risk of silent data corruption, it is good practice to periodically perform a full data rewrite.

A simple read-only pass over the device can help detect obvious failures, but it does not meaningfully improve long-term retention:

pv /dev/sdX > /dev/null

This command safely reads the entire device but does not refresh the stored data.

To actually mitigate data decay, the data should be rewritten. A practical approach is:

• Copy all data off the SD card.
• Reformat the card.
• Copy the data back onto the card.

This process forces the controller to reprogram all memory cells and remap any weak blocks. Performing this maintenance every 1–2 years, combined with multiple independent backups stored on different media, significantly reduces the risk of long-term data loss.

In short: flash memory is reliable, but not timeless. Long-term preservation requires active maintenance, not just storage.