What is an SD Card?
SD cards, or Secure Digital cards, are a type of memory card used in cameras, phones, and other portable devices to store media files like photos and videos. The SD standard was introduced in 1999 by Panasonic, SanDisk and Toshiba as an improvement over MultiMediaCards (MMC). The first SD cards held up to 2GB of data (Digital SD Card Breakdown).
SD cards have evolved significantly over the years with increases in capacity and speed. Modern SD cards are available in capacities ranging from 4GB to 1TB. There are also different classes and form factors of SD cards like SDHC, SDXC, microSD, and miniSD (MicroSD Cards). SD cards use flash memory and have no moving parts, making them more compact and durable than mechanical hard drives.
SD Card Capacities
SD cards come in a range of storage capacities to meet different storage needs. The most common SD card capacities available today are:
- 4GB
- 8GB
- 16GB
- 32GB
- 64GB
- 128GB
- 256GB
- 512GB
Lower capacity cards like 4GB and 8GB are sufficient for basic usage like storing documents or small media files. Higher capacity cards in the 64GB to 512GB range are better suited for high resolution photos, 4K video recording, gaming, and other storage intensive uses.
According to source, the most common SD card capacities typically range from 32GB to 256GB for average consumers. However, capacities up to 512GB are now available as technology continues to improve.
How SD Card Storage Works
SD cards use flash memory to store data. This type of memory is non-volatile, meaning it retains data even when power is turned off. The flash memory in SD cards is arranged in blocks, typically 512 bytes to 4 kilobytes in size. When files are saved to the SD card, they are split into chunks that fit into these blocks (Minitech, 2022).
The card uses a file allocation table (FAT) to keep track of which blocks contain data for each file. The FAT acts like a table of contents, storing the addresses of each block making up a file. This allows the files to be accessed, written, deleted and recovered. The FAT system also manages free space, keeping track of unused blocks available for new data (ProGradeDigital, 2023).
Overall, the combination of flash memory blocks and the file allocation table allow SD cards to reliably store files digitally in a compact removable format. This makes them highly useful for portable devices like cameras, phones, and handheld gaming systems.
Sources:
Minitech. (2022). How Do SD Cards Work and How to Use Them on Different Devices. https://www.minitool.com/partition-disk/how-do-sd-cards-work.html
ProGradeDigital. (2023). How Do Memory Cards Work? https://progradedigital.com/how-do-memory-cards-work/
Why Full Capacity Isn’t Shown
There are a few key reasons why an SD card doesn’t show its advertised full capacity when you insert it into a device or view its properties:
File System Overhead
SD cards come formatted with a file system like FAT32 or exFAT, which use some space to store data about the files on the card such as filenames, timestamps, folder structures and more. This file system overhead takes up a portion of the total advertised space – typically around 5-10% 1.
Pre-Formatting
Manufacturers may pre-format SD cards before packaging and sale, using up some storage capacity. Formatting an SD card yourself after purchase can restore this lost capacity 2.
Bad Sectors
SD cards can develop bad sectors over time that become unusable for storage. The operating system marks these bad sectors as unavailable, resulting in lost capacity. Low-level formatting tools can attempt to restore them 3.
File System Overhead
One of the main reasons why an SD card doesn’t show its full capacity is due to file system overhead. The file system on the SD card uses some of the available space to store data needed to organize and manage files on the card.
For example, FAT32 which is a common file system for SD cards, uses a file allocation table (FAT) to keep track of used and unused storage clusters. The larger the capacity of the SD card, the larger this FAT needs to be, taking up more space. Typically around 2-4% of an SD card’s capacity can be used up just by the FAT alone depending on the size of the card [1].
Other file system structures like the root directory and inodes for keeping track of files also require storage space. So in total around 5-10% of the stated capacity of an SD card may be reserved just for the file system’s overhead data.
Pre-Formatting
When an SD card is manufactured, it goes through a pre-formatting process to prepare it for use. This pre-formatting creates a file system and performs a bad sector check, mapping out any bad sectors found so they won’t be used for data storage.
The pre-formatting process uses up some of the card’s total capacity. Typically, anywhere from 100MB to 1GB of space can be used just for storing this pre-format data [1]. The more bad sectors identified, the more space will be allocated for this overhead data.
So out of a 32GB card, the actual usable capacity may be 31GB or less straight out of the package. This unallocated space is unavailable to the user and accounts for some of the missing capacity when first using the card.
Bad Sectors
SD cards can develop bad sectors through prolonged use. Bad sectors are portions of the storage that can no longer reliably store data due to physical damage or manufacturing defects. When your device writes data to a bad sector, the data can become corrupted or lost entirely (Minitool).
Bad sectors often increase over time with normal use of an SD card. The more a card is written to, the more likely bad sectors will appear. Signs of bad sectors include frequent data corruption, SD card errors, or significantly reduced available space. Using tools like CHKDSK can detect and isolate bad sectors (Recoverit). While bad sectors cannot be repaired, your device can mark them as unusable so data is no longer written to those areas.
To maximize lifespan, avoid overfilling your SD card and repeatedly writing/deleting files. Back up data periodically in case bad sectors develop. Replacing cards after prolonged heavy use can prevent widespread data loss from worsening bad sectors.
Formatting the Card
One factor that can impact how much capacity is available on an SD card is the choice of filesystem used to format it. SD cards commonly come pre-formatted with the FAT32 filesystem, which has some limitations. Notably, FAT32 has a maximum file size limit of 4GB, and it is not as space efficient for larger capacity cards, meaning more of the available space is consumed by filesystem overhead. To gain back some lost capacity, the card can be reformatted to a more modern filesystem like exFAT or ext4 that have less overhead. According to a helpful forum post (https://www.briskoda.net/forums/topic/431125-direct-download-links-for-ece-20172018-maps/), reformatting to exFAT allowed the full capacity to be utilized when the original FAT32 format showed less space. Just be aware that reformatting will erase all existing data.
Checking Capacity
When you first get an SD card, you may notice that the full advertised capacity is not showing up when you check on your computer or device. There are a couple easy ways to check the actual usable capacity.
On Windows, you can check the properties of the SD card drive in File Explorer. This will show the total capacity and free space available. On Mac, you can view the capacity in Finder under General information.
For mobile devices like phones and cameras, you can go into the storage settings to see the details of your SD card capacity and free space. On Android, go to Settings > Storage. For iPhone, go to Settings > General > iPhone Storage.
There are also free utilities and apps you can download to get more detailed SD card information. SD Card Formatter for Windows will analyze the card and show full details. On mobile, SD Card Test or SD Card Analyzer apps can be used.
Checking the actual capacity on your computer or device is the best way to verify if the SD card matches the advertised size. This also lets you see how much free space is really available to use.
Improving Capacity
There are a few tips to minimize the loss of capacity on an SD card:
Use the exFAT file system instead of FAT32. exFAT has less overhead which allows more storage capacity. However, exFAT may not be compatible with all devices [1].
Avoid pre-formatting the SD card. Pre-formatted cards reserve space for system files which can’t be used for storage. Formatting the card yourself allows full use of the capacity [2].
Check for and mark bad sectors. Bad sectors take up space but can’t be used for storage. Marking them allows the capacity to be recalculated without them.
Minimize the number of small files. Small files have more metadata overhead. Storing data in larger files makes more efficient use of capacity.
Choose lower resolution/bitrate if possible. Lower resolution video and images take up less space per file.
Regularly reformat the card. As files are deleted and written over, fragmentation builds up over time, wasting space.