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There are three main types in the SD memory card family. SD, SD High Capacity (SDHC™), and SD Extended Capacity (SDXC™). This article details the different specifications of all three types of SD memory cards and the speed class ratings and compatibilities that are different with each type.

Additional Reference: SD Assocation Consumer Information

SD capacities range from 128MB to 2GB
Default Format: FAT16
SD cards will work in all host devices that support SD, SDHC, or SDXC

SD High Capacity (SDHC™) card is an SD™ memory card based on the SDA 2.0 specification.
SDHC capacities range from 4GB to 32GB
Default Format: FAT32
Because SDHC works differently than standard SD cards, this format is NOT backwards compatible with host devices that only take SD (128MB - 2GB) cards. Most readers and host devices built after 2008 should be SDHC compatible.
To ensure compatibility, look for the SDHC logo on cards and host devices (cameras, camcorders, etc.)

SD (Secure Digital) SDHC (Secure Digital High Capacity) SDXC (Secure Digital Extended Capacity) non-UHS (Ultra-High Speed) UHS-I; UHS-II; This card reader is a plug and play device and works on all operating systems. The IOGEAR Thunderbolt 3 6-Slot SD Card Reader is built to suit the needs of professionals who prefer to work outdoor. Secure Digital Extended Capacity (SDXC) Secure Digital Extended Capacity (SDXC) microSDHC microSDHC. Flash Memory Card Flash Memory Card. Water Resistant.

Secure Digital (SD) cards are flash memory cards that comply with standards set by the SD Association. Hemera/ Thinkstock Many laptops and mobile devices you can buy today have built-in slots for Secure Digital memory cards, usually just called SD cards. Secure Digital, officially abbreviated as SD, is a proprietary non-volatile memory card format developed by the SD Association (SDA) for use in portable devices.

SD Extended Capacity (SDXC™) card is an SD™ memory card based on the SDA 3.0 specification.
SDXC capacities range from 64GB to 2TB
Default Format: exFAT
Because SDXC uses a different file system called exFAT and it works differently than standard SD cards, this format is NOT backwards compatible with host devices that only take SD (128MB to 2GB) or host devices that only take SDHC (4GB to 32GB). Most host devices built after 2010 should be SDXC compatible.

To ensure compatibility, look for the SDXC logo on cards and host devices (cameras, camcorders, etc.).

NOTE: Internal card readers on laptops up to year 2008 may NOT support SDXC cards. SDXC cards will work in SDHC compatible readers (not SD readers) if the computer OS supports exFAT.
For more information on exFat see: Operating Systems that support the exFAT File System

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Ultra High Speed, Phase I (UHS-I) bus design for SDHC and SDXC cards was added in SD spec 3.0. This is a design enhancement to increase the performance of SDHC/SDXC cards.
UHS-I specification defines two bus architecture options for up to 50MB/s (UHS-50) and 104MB/s (UHS-104) data transfer rates. These are theoretical maximum limits and actual maximum performance for a specific card is defined on it label or in advertising.
UHS is an enhancement to the original SD interface specifications. Host devices will obtain the UHS maximum speed when both the card and host device support UHS. Otherwise, the host device and card will use the slower SD maximum speed obtainable.
There is no compatibility problem using a UHS card with a non-UHS device.

Ultra High Speed Phase II (UHS-II) bus design for SDHC and SDXC was added in SD spec 4.0. This is an additional design enhacement with connector interface modifications to increase performance.

UHS-II specification defines bus architecture for options of 156MB/s and 312MB/s. Manufacturers implemented the 312MB/s option in memory cards. These are theoretical maximum limits and actual maximum performance for a specific card is defined on the label or in advertising.

Host devices will obtain the maximum UHS-II speed when both the card and the host device support UHS-II. Otherwise, the host device and card will use the highest compatible UHS-I or SD bus speed.

There is no compatibility problem using a USH-II card in a UHS-I or non-UHS device.

Speed Class & UHS Speed Class Rating
Speed class ratings define a minimum guaranteed speed of SDHC/SDXC cards. Speed class rating is important for HD video mode or camcorders, where the device is actually saving a steady stream of data. The resolution and format of the video determines the amount of steady stream data. You should consult your camera user manual for minimum speed class requirements for HD video modes.

Speed Class (SD Bus)

UHS Speed Class (UHS Bus)

For a more detailed explanation of speed class see: Difference between Speed Class, UHS Speed Class, and Speed Ratings...
SD Association: Speed Choices

Compatibility Summary
The chart below shows the host device and the memory cards supported.

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What is SD?

SD describes devices that conform to SD standards for non-volatile memory cards. See the SD Association website for details.

Are there size limitations for the cards that can be inserted into the SD slot?

Yes. The SD card specification for a memory card is 32mm by 24mm by 2.1mm. You can also use thinner cards, such as MultiMediaCards (MMC). Avoid using cards that have a thickness greater than 2.1mm, as they might damage the SD card slot if you try to insert them.

Which SD card formats work in the SD card slot?

Cards that conform to the SD 1.x, 2.x, and 3.x standards should work. The SD card slot can use:

• Standard SD, 4MB to 2GB
• SDHC (Secure Digital High Capacity), 4GB to 32GB
• SDXC, 4GB to 2TB
• MMC (MultiMediaCards)
• UHS‑II, up to 2TB (iMac Pro only)

You can use a passive adapter like the one shown here to make MiniSD, MicroSD, and higher density formats like MiniSDHC and MicroSDHC conform to the width and thickness specifications listed above:

How fast can my Mac read or write to an SD card in the SD card slot?

Mac notebooks use the USB bus to communicate with the SD card slot. They have a maximum speed of up to 480Mbit/s. Mac desktops use the PCIe bus to communicate with the SD card slot. Desktops can transfer data at a faster rate.

Check the packaging that came with your SD media to determine the maximum transfer rate that your specific card uses.

To determine the maximum transfer speed of your Mac, you can use System Information. Choose Apple () menu > About This Mac and then click System Report.

If you use a Mac notebook:

1. Select Hardware, then select USB.
2. Select Internal Memory Card Reader and look for the Speed entry.

If you use a Mac desktop computer:

1. Select Hardware, then select Card Reader.
2. Look for the Link Speed entry. Computers that use the PCIe bus express their speed as GT/s.

Does the SD slot work with cards that exceed 32GB?

Yes. However, most media manufacturers preformat the media using common block-and-cluster sizes that don’t approach the theoretical limits of a given file system.

Most SD cards use the FAT32 file format, and preformatted FAT32 SD media is commonly available up to a capacity of 32GB. Media that exceeds 32GB usually uses the exFAT file system, while some smaller capacity cards use the FAT16 file format. Preformatted FAT16 media is generally available up to a capacity of 2GB.

If you use OS X Snow Leopard 10.6.5 or later, you can find out which file system you’re using:

1. Insert the media into the SD card slot.
2. Choose Apple menu > About This Mac.
3. Click System Report.
4. In the Hardware section, click Card Reader, and find the File System field.

Will the SD card slot work with SD cards that use the exFAT file system?

Yes. Any Mac that has an SD card slot and is running OS X 10.6.5 or later can use the exFAT file system.

exFAT is also supported in Boot Camp with Windows 7, 8.1, or 10 on any Mac made in 2011 or later with an SD card slot.

How do I insert media into the SD card slot?

When you insert the card, make sure that the metal contacts face down and point toward the computer. Don’t force media into the SD card slot, as this might cause damage.

How does my Mac use the media inserted into the SD card slot?

Your computer recognizes a card inserted into the SD card slot as a USB storage device. You can mount, read from, and write to the SD card just like you can with any other USB storage device.

I put the card in the slot, but it didn’t mount. What should I do?

Remove the card and insert it again. Sometimes the SD card won’t mount properly if you put it into the slot too slowly.

When I try to write content to the card, I get a 'cannot be modified' message. How can I fix this?

You see this message when you try to edit data on an SD card that’s locked. You need to use the lock slider to unlock the card before you can edit the data.

To eject the card, drag the icon that represents the card to the Trash. After the icon disappears from the desktop, you can remove the card from the computer. Adjust the lock slider tab to unlock the card, then reinsert the card into the slot. See the manufacturer’s instructions for the location of the slider tab.

Can I use Disk Utility to reformat an SD card?

You can use Disk Utility to partition and format an SD device as FAT32 (using the MS-DOS FAT setting) or Mac OS Extended. The Mac OS Extended format can be used only on Macintosh systems. Non-Apple systems won’t recognize cards formatted to Mac OS Extended.

You might have to format a card that’s larger than 32GB with exFAT if you want to use it with a digital camera, GPS, or another device. When in doubt, format the card in the device that you intend to use it with.

Can I install macOS on an SD storage device and use it as a startup volume?

Use Disk Utility to change the default partition table to GUID. Then format the card to use the Mac OS Extended file format.

How do I remove a card from the SD card slot?

Before you remove the card, allow any data transfer to SD media to complete. To eject the card, drag the icon that represents the card to the Trash. After the icon disappears from your desktop, you can remove the card from the slot.

Don't remove a card while your Mac is sleeping, as this could lead to data loss. Always wake your computer and eject the SD card before removing it from your Mac.

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Can I use Secure Digital Input Output (SDIO) cards?

Can I use macOS to see the specifications for the interface hardware and media inserted in the SD card slot?

You can get information about the interface hardware and the media that you inserted in the slot from the System Information:

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1. Choose Apple menu > About This Mac.
2. Click System Report.
3. In the Hardware section of System Information, select USB.
4. In the list of USB devices, select Internal Memory Card Reader to access information about the interface hardware and the media inserted into the SD card slot.

Can I use the SD card slot while running Windows using Boot Camp?

The SD card slot works with Boot Camp in all supported versions of Windows. You’ll need to download and install the Windows Support Software to use the SD card slot with Windows.

Learn more about using Windows on a Mac.

Can I use an SDXC card on my Mac with Windows?

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You can use an SDXC card in Windows 7, Windows 8, and Windows 10 with these Mac computers:

• MacBook Pro (Early 2011 and later)
  MacBook Pro models from 2016 and later don't have a built-in SD card slot. You can use a USB-C card reader, or a combination of a USB-C to USB Adapter and a USB card reader.
  
• MacBook Air (Mid 2011 and later)
• Mac mini (Mid 2011 and later)
  Mac mini (Mid 2010) doesn't support SDXC cards.
• iMac (Mid 2011 and later)
  iMac (Mid 2010) doesn't support SDXC cards.

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Can I use an SD, SDHC, or SDXC card to install Windows on my Mac?

No. You can’t use SD, SDHC, or SDXC cards with Boot Camp to install Windows software on Mac computers.