There is an excellent Support Article on Apple's website that addresses this: About the SD and SDXC card slot
Does the SD slot work with cards that exceed 32 GB?
Yes. However, most media manufactures preformat the media using common
block-and-cluster sizes that do not approach the theoretical limits of
a given file system. Most SD cards use the FAT32 file format.
Preformatted FAT32 SD media is commonly available up to a capacity of
32 GB. Media that exceeds 32GB usually uses the exFAT file system.
Some smaller capacity cards use the FAT16 file format. Preformatted
FAT16 media is generally available up to a capacity of 2 GB. To
determine which file system you are using with Mac OS X v10.6.5 or
later, insert the media into the SD card slot, go to the Apple menu,
select About this Mac..., click on More info..., click on Card Reader
in the hardware section, and locate the File System field.
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 cards that are Standard SD (Secure Digital) 4
MB to 2 GB, SDHC (Secure Digital High Capacity) 4 GB to 32 GB, and
SDXC (Secure Digital Extended Capacity) 4G B to 2 TB. MMC can also be
used in this slot. MiniSD, MicroSD, and higher density formats like
MiniSDHC and MicroSDHC can also work with the use of "passive"
adapters that conform to the width and thickness specifications listed
To format your SD card, you should use Disk Utility either using the GUI or via Terminal.
diskutil eraseDisk HFS+ DiskName GPT /dev/diskN
HFS+ = type of filesystem
DiskName = Name you want to give the SD card (i.e. "Stuff", "My Photos")
GPT = partition type
diskN = disk identifier (i.e disk4, disk7, etc.)
For more details, see