Macs can read and write to Secure Digital cards formatted for almost any device, but you may want to reformat an SD card, either to change its compatibility or to erase everything and start over.
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Formatting the SD Card
Insert the card into an external card reader connected to your Mac. Its name should appear on the desktop.
Copy any data you need onto the Mac's hard drive. Formatting erases the card; you can copy it back onto the card afterward.
Open the Disk Utility application located in Applications > Utilities or use Shift-Command-U from the Finder.
Select the SD card from the list of mounted drives in the column on the left. Select the card itself -- not any mounted partitions, which appear indented below it if they exist.
Select Erase from the options across the top of the main part of the window. "Erase" means "format" in this case.
You can also select Partition. This formats the card's total storage capacity into separate directories, each of which appears as a separate card on the Mac's Desktop. Partitioning is not a good idea for cards that are going to be used on Windows or Android devices; they may only see the first partition.
Choose the desired format from the drop-down menu. Select OS X Journaled for an OS X bootable card, MS-DOS (FAT) for Android or Windows 95 and later, or exFAT for Windows Vista SP1 and later.
Click the Erase button beneath the drop-down menu, confirm, and wait for the card to format.
Choosing a Format
The main reason to format an SD card specifically for Mac OS X is to create a bootable startup disk, the one containing OS X that you run from. This may be for troubleshooting, restoring from a Time Machine backup, or running programs that are only compatible with an older version of Mac OS X.
Mac OS X can format SD cards in the Windows-compatible FAT and newer exFAT formats. The advantages to FAT are that Windows 95 and XP can read it, and also Android devices tend to be happier with it. The exFAT format was introduced the same time as Windows Vista SP1, and it's a more efficient format for SD cards and other external media that access large files -- the files use less space on the card.
There are several generations of Secure Digital -- SDHC, SDXC, microSD and miniSD -- and more may follow. While all SD readers, including the ones built into some Macs, are backward compatible, you may still need a current reader. These USB devices are inexpensive and readily available.
Alternatively, you may be able to use a microSD-to-standard SD adapter. These are also inexpensive, and they often come with microSD cards to ensure they'll work in older readers.