SDHC, SDXC, SDUC, Oh My!

The complexity of the variety of Secure Digital (SD) Cards continues to confuse me, in particular the various card standards:  SD, SDHC, SDXC, and SDUC.

SD Cards are so important nowadays.  They've become primary storage for many applications, like embedded applications and single-board computers (SBC), e.g., Raspberry Pi.  They've replaced floppy disks, Zip disks, etc. as "sneaker net" media.  And of course their role in digital cameras of all types.

Therefore it is helpful to know about all these standards, especially since SD card readers are NOT forwards compatible to the newer standards.  For example, you cannot take an SDXC card and use it in a card reader that only supports SD and SDHC.  I've coalesced the following information from various sources to provide a high level timeline of when these cards were introduced and their capacities.  (for information about SD Card speed classes, which is an independent concept from SD Card capacity see:  New SD Express Speed Classes)

1999

• original SD Card specification
• called either "SD" or "SDSC" for SD Standard Capacity
• supported capacities:  4MB, 8MB, 16MB, 32MB, 64MB, 128MB, 256MB, 512MB and 1GB, 2GB

2006

• "SDHC":  SD High Capacity
• supported capacities:  4GB, 8GB, 16GB, 32GB

2009

• "SDXC":  SD eXtended Capacity
• supported capacities:  64GB, 128GB, 256GB, 512GB, 1TB, 2TB

2019

• "SDUC":  SD Ultra Capacity
• supported capacities:  4TB, 8TB, 16TB, 32TB, 64TB, 128TB

Remember:  As depicted in this image, SD Card readers can read media of the same standard or media of previous standards.  SD Card readers cannot read media of subsequent standards.