The SD Association recently took the wraps off of its latest SD 7.0 specification for SD cards which brings huge increases in potential capacities and data transfer speeds with the introduction of SD Ultra Capacity (SDUC) and integration of PCI-E and NVMe technologies for SD Express respectively. Teaming up with PCI-SIG (the group behind the PCI/e specifications), future SD cards will be available as SDUC or SD Express or both (cards can be SDUC but not SD Express and vice versa). The SD Ultra Capacity specification boosts the capacity limit from 2 TB (SDXC) to 128 TB while SD Express repurposes the second row of pins in UHS II cards to support a PCI-E 3.0 x1 connection along with NVMe 1.3 interface and its SSD-friendly features like bus mastering, multi queue (sans locking), Host Memory Buffer, NVMe power control, and other memory access mechanisms. The PCI-E interface enables up to 985 MB/s transfers for SD Express compatible hosts while retaining backwards compatibility (at slower speeds over the SD interface) with other/older hosts.
SDUC cards will be available in micro SD and SD flavors while SD Express will be available initially in full size SDHC, SDXC, and SDUC cards (denoted with a new EX I suffix). SD 7.0 specifies SD cards using two power supplies (3.3v and 1.8v) with pin #18 on the second row being reserved for a future 1.2v supply for lower power cards/modes. SD Express cards can be initialized by the host using either SD or PCI-E interfaces, though using the legacy SD interface is recommended. When operating in PCI-E / NVMe mode, the card using standard NVMe drivers and shows up as a typical NVMe device to Windows. The new interface supports most SD card features such as password locking and write protection, but SD-CPRM Security (the DRM used on SD cards) and speed classes do not work over the PCI-E interface.
Because SD Express uses the second row of pins that are used in current UHS-II cards, SD Express cards will not be able to operate in UHS-II mode on UHS-II hosts. UHS-I remains unaffected, however.
In all the new specification sounds great and will help future SD cards remain relevant in mobile devices as well as high end professional and consumer video equipment especially as the industry moves towards 8K and beyond for production and consumers are recording and watching more 4K+ high bit-rate content than ever before. The PCi-E 3.0 and NVMe 1.3 interfaces (up to 985 MB/s) should provide competition for high end video cameras like those from RED that use removeable solid state drives with similar speed feats in a smaller form factor. The SD Association further mentions applications in automotive and VR applications for SD Express.
There are still questions as far as how much the hardware (PHY and NVMe controller) needed to enable SD Express will add to the cost of SD cards and how long it will be until cards with limited numbers of flash can actually start to approach those maximum data transfer speeds and capacities.