SD cards are getting bigger and faster. The new SD Express standard greatly increases maximum capacity and speed. SD Express cards are backward compatible with your current hardware, but you need new hardware for the highest speed.
SD card reaches its current limits
Since we use SD and microSD cards in 3D cameras, more powerful smartphones, DSLR cameras, tablets, game consoles and even cars, there are ever increasing demands on storage space and faster read and write speeds.
SanDisk recently announced what it was called the world's fastest 1TB microSD card, with a read speed of 160 MB / s, which is enough to "transmit 1000 high resolution images and 30 minutes 4K video (24GB) in less than 3 minutes . " Some cards have lower storage capacity but higher write speeds, such as the Delkin 128 GB microSD card, which has the right speed at 300 MB / s. However, manufacturers are already running the limit on current standards. For example, the theoretical limit for SDXC is 2 TB.
SD cards get faster speed SSD protocols
Through the incorporation of PCIe (peripheral component interconnect printer) and NVMe protocols (non-volatile memory express), SD cards have a new theoretical limit of 985 MB / s, more than six times as fast as the 1 TB SanDisk cards. If PCIe and NVMe are familiar to you, it is because they have been used for some time SSD files and SD Express cards can act as removable solid state drives.
The SD association says that these new speeds will allow for super slow motion, 8K video and add straightforward burst mode support, which will be a blessing to digital photographers. In addition, SD Express cards, by integrating PCIe 3.1, can consume less power than the cards came before it. In theory, this should contribute to better battery life on mobile products, however much it is unclear.
In summary, the idea is that utilization of existing technology will accelerate adoption, as new testing equipment and development processes would not be needed. Manufacturers can use what they already know from creating solid state devices.
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As part of the transition, SD and microSD card size increase, increasing the limit from 2TB to 128TB. Cards that have capacity for more than 2 TB will be marked as SDUC, adding SDHC and SDXC categories.
As always, reading and writing speeds are separated from storage capacity, so you can see SDHC and SDXC cards that are marked as SD Express and can speed up writing speeds. And all these benefits come in both SD and MicroSD cards, and SD Express cards are backward compatible with previous devices.
They are backward compatible, but you lose speed
All this amazing speed is partly due to a new line of pins available on SD Express (and SD UHS-ii) cards. Unfortunately, this means that your current devices cannot use these pins, they lack the necessary hardware. But according to the SD association, you still get the extra storage space.
Because the association explains it in its white paper, the card and device may not be able to achieve their best performance, but the consumer will have access to all the content. SD Express continues this long-standing promise by keeping the ability to run the new cards into billions of existing products available on the market through its old SD interface. "
When can I buy one and how much will it cost?  There are no microSD cards with the" Express "label in the stores yet. The SD association, which is the industry group that monitors this standard, announced it The new microSD Express standard at Mobile World Congress 2019 on February 25, 2019. It is part of the new SD 7.1 specification The SD Express standard was published on June 26, 2018, as part of the SD 7.0 specification.
The SD Association did not announce any launch date when microSD Express cards would be available from hardware manufacturers and did not have any SD Express cards on the market since the announcement back in June. We are not sure when they will be available, which manufacturers will make them, or how much they will cost, nor do we know when devices that are compatible with this new standard will be available to fully exploit Well the higher speeds, all we know is the standard is out and units will eventually start to take advantage of it.