SD cards are a popular form of portable storage for devices like cameras, phones, and tablets. They use non-volatile flash memory to store data, which means the data remains even when the SD card loses power. This makes SD cards useful for transporting and accessing files across devices.
When an SD card is not connected to a host device like a computer or phone, it is completely unpowered. Even in this state, the flash memory retains all previously saved data. This gives SD cards the ability to store data indefinitely with no external power source.
But how long can this data actually persist? The retention or shelf life of an SD card can vary substantially based on factors like memory type, storage conditions, and write/erase cycles. Typically, manufacturers claim anywhere from 1-10 years of data retention if stored properly.
In this article, we’ll explore how SD cards are able to preserve data when unpowered, examine what affects the shelf life, and provide tips for maximizing data retention.
SD Card Basics
SD cards use flash memory to store data. Flash memory stores data in tiny memory cells on an integrated circuit. These memory cells act as gates or switches that can be either ON or OFF to represent 1s and 0s of binary data (Progradedigital.com, 2023).
When data is written to an SD card, it alters the electrical charge in the memory cells to set them to either ON or OFF states. When data is read from the card, the electrical charge of the cells is sensed to determine if they are ON or OFF, allowing the 1s and 0s to be interpreted by the device (Quora.com, 2015).
SD cards have a controller chip that manages the writing and reading of data to the flash memory cells. The controller also handles things like security, encryption, and interfacing with the device using the SD card. Overall, the flash memory provides storage capacity, while the controller chip handles data management.
DRAM vs Flash Memory
DRAM (Dynamic Random Access Memory) and Flash are two different types of semiconductor memory with key differences in how they store data. The main difference between DRAM and Flash in terms of retaining data without power is that DRAM is volatile while Flash is non-volatile.
DRAM requires power to maintain the stored data. It stores each bit in a separate capacitor within an integrated circuit. The capacitor can be either charged or discharged; its charged state represents a “1” and its discharged state represents a “0.” But these capacitors gradually leak their charge, so the stored data needs to be refreshed periodically, usually thousands of times per second. As soon as power is removed, the capacitors discharge and all data in DRAM is lost1.
In contrast, Flash memory is non-volatile, meaning it retains stored data even when not powered. It stores data in an array of memory cells made from floating-gate transistors. Each cell has a floating gate which holds an electrical charge to represent one bit. The charge remains in place even when power is removed, allowing Flash to store data indefinitely. Unlike DRAM, Flash doesn’t require constant refreshing. This makes Flash suitable for long-term data storage applications2.
In summary, DRAM offers faster read/write performance but requires power to maintain data. Flash is slower but its non-volatile nature allows it to store data for extended periods without power.
Flash Memory Types
SD cards utilize flash memory, which stores data in memory cells made up of floating-gate transistors. There are two main types of flash memory used in SD cards: NOR flash and NAND flash. NOR flash is faster for reading data but slower for erasing and writing data. NAND flash has faster erase and write speeds but slower access times (1). SD cards use NAND flash memory due to the faster speeds and higher storage density. Within NAND flash, there are several types of SD cards that offer different capacities and speeds:
- SD (Secure Digital): The original SD card with up to 2GB storage capacity.
- SDHC (Secure Digital High Capacity): Introduced additional storage from 4GB up to 32GB.
- SDXC (Secure Digital Extended Capacity): Allows for SD cards from 64GB up to 2TB storage capacity.
- SDIO (Secure Digital Input Output): Combines SD card interface with input/output functions.
- microSD: A smaller form factor of the SD card popular in mobile devices.
Each SD card type utilizes the same NAND flash memory technology but expands on capacity and performance. Newer versions maintain backwards compatibility with older host devices (2).
(1) https://www.techtarget.com/searchstorage/definition/memory-card
(2) https://www.kingston.com/en/blog/personal-storage/microsd-sd-memory-card-guide
Data Retention
The data retention or shelf life of an SD card refers to how long the data can be stored and reliably retrieved from the card without deterioration. This depends on the type of flash memory used in the card.
For SLC (single-level cell) flash memory, which stores 1 bit per memory cell, data retention is typically 10 years or more at room temperature (
Therefore, SLC flash offers the longest data retention times, while TLC has the shortest. However, higher density flash like TLC allows for greater storage capacities in SD cards.
Real-World Examples
There are some real-world examples of data being recovered from unpowered SD cards over long periods of time. According to one source, a study by Delkin Devices found that SD cards stored at room temperature could maintain data integrity for up to 2 years without power. However, after 4 years, some data corruption started to occur. Another user reported successfully recovering photos from an SD card that had been unused for 5 years.
Overall, while data may be recoverable from SD cards after a few years without power, there are no guarantees. The longer the card goes unpowered, the higher the risk of data loss. For reliable long-term storage, it is recommended to transfer data from SD cards to other media and store in cool, dry conditions.
Effects on Data Retention
There are several factors that can affect how long data is retained on an SD card without power:
Temperature – Higher temperatures can degrade flash memory and reduce data retention over time. According to the SD Association, the ideal storage temperature for SD cards is between -40°C to 85°C. Storing cards above 85°C for extended periods may impact data retention. For example, leaving an SD card in a hot car on a summer day may cause data loss over time.1
Physical Damage – Any physical damage to the card from drops, bends, water exposure, etc. can affect the flash memory and ability to retain data. The metal casing helps protect the components, but physical damage can still occur and degrade data retention.
rewrite cycles – The flash memory cells in SD cards have a limited number of rewrite cycles before they wear out. Higher quality cards are typically rated for thousands of rewrite cycles, but frequent writing/deleting files will shorten the overall card lifespan and data retention.
File System Corruption – If the file system on the SD card becomes corrupted for any reason, the stored data can become inaccessible and unreadable even if electrically intact.
Bit Rot – Over many years, background radiation and read/write operations can randomly flip bits stored on flash memory. ECC and wear leveling technologies help minimize this, but bit rot over decades may still randomly corrupt data.
Overall, cool and stable temperatures, minimal rewrite cycles, avoiding physical damage, and periodic data refresh help maximize long-term data retention on SD cards without power.
Maintaining Integrity
When storing data long-term on SD cards, it’s important to take steps to maintain the integrity of the data. Here are some tips:
Use high-quality, name-brand SD cards from reliable manufacturers like SanDisk or Samsung. Avoid cheap, off-brand cards which are more prone to errors and failures over time.
Periodically refresh the data stored on the cards. Every few months, copy the data off the card onto a computer or other storage device, reformat the card, and then copy the data back. This helps prevent data degradation.
Store SD cards properly in a cool, dry, dark place when not in use. Avoid exposing them to extreme heat, moisture, sunlight, or electromagnetic fields which can damage data.
Use error-checking software like h2testw to scan for errors and bad sectors, replacing any cards that fail the test.
Make multiple backups of important data stored on SD cards onto other devices. This provides redundancy if any single card fails.
Handle SD cards gently and properly, avoiding physical damage. Damaged cards are prone to data loss and corruption over time.
Other Data Storage
While SD cards are commonly used for removable data storage, there are alternatives that may be better suited for long-term archival needs. One such option is optical discs like CDs, DVDs, and Blu-ray discs.
Optical discs have some advantages for archival storage. They are read-only media, so the data cannot be accidentally deleted or overwritten. The discs are also fairly durable if stored properly and data can persist for decades. For example, high-quality CD-R and DVD-R discs are rated to last 50-100 years before data loss occurs.
Optical discs also offer very large storage capacities. A single-layer Blu-ray disc can store up to 25 GB, while dual-layer discs store up to 50 GB. This makes them suitable for backing up large amounts of data for archival purposes. Many Blu-ray recordable discs also feature an inorganic data layer that is resistant to degradation.
The downside is that optical disc drives are no longer standard on computers. So accessing archived data on optical media may require an external drive. The discs are also sensitive to light, heat, and physical damage if not stored properly. But as a whole, optical discs represent a proven archival storage medium that has advantages over volatile media like SD cards.
Conclusion
In summary, how long an SD card can retain data without power depends on several factors. The type of NAND flash memory impacts retention, with SLC lasting longer than MLC, TLC, QLC. Temperature affects retention, with higher temps degrading data faster. The age of the card matters too, as older cards lose integrity over time. Generally, SD cards can hold data without power for 1-10 years if stored properly. To maintain data, store cards in cool, dry places, refresh data periodically, and use enterprise-grade cards for archival purposes.