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Best SD card | Buying Guide : Things to consider while buying a SD card

With this guide, we’ll break down what all the different codes and ratings mean, and offer the best choices for a range of categories.

Size & Storage

All SD cards (short for Secure Digital), regardless of their size, use one or two small NAND flash memory chips — similar to those found in USB memory sticks and SSDs — and a tiny processor to manage the flow of data and instructions.

There are 3 standards for the dimensions, and they are incompatible with each other. In other words, a miniSD card reader won’t work with microSD cards (unless you use an adapter):

• Standard SD cards: 1.26 x 0.94 x 0.083 to 0.055 inches (32 x 24 x 2.1-1.4 mm)
• miniSD cards: 0.85 x 0.79 x 0.055 inches (21.5 x 20 x 1.4 mm)
• microSD cards: 0.56 x 0.43 x 0.039 inches (15 x 11 x 1 mm)

Standard SD cards all come with a small locking toggle, that enables/disables the ability to write or delete data on the card; however, mini and microSD cards don’t have this feature. There’s also a further 5 categories within the size classes, that indicate the connection system and data capacity of the card:

• SD or SDSC (Secure Digital Standard Capacity): maximum storage of 2 GB
• SDHC (Secure Digital High Capacity): More than 2 to 32 GB of storage
• SDXC (Secure Digital Extended Capacity): More than 32 GB to 2 TB of storage
• SDUC (Secure Digital Ultra Capacity): More than 2 to 128 TB of storage

The fifth category, Secure Digital Input Output (SDIO), are special in that they contain more than just storage. These cards sport an extra device that provides additional functions, such as a Bluetooth or GPS receiver. Because there is a big difference in the storage sizes, each category also has restrictions of the file format used.

Speed Classes

All SD cards use the little brass contacts at the end of the package to receive and send information, in the form of instructions and data. The interface between the card and the reading device has evolved with each specification revision — in some cases, the updated system just runs faster (i.e. the bus clock is higher) but in some cases, the SD card has extra contacts to provide more channels for the data.

The differences between all the interfaces and performance are set out in so-called speed classes, and each one is generally organized by the peak bus throughput . This is a measure of the maximum amount of bytes per second that can be transferred between the SD card and host device.