To Format SD card to FAT32 in Mac OS, follow below instructions:
1. Connect the SD card to the Mac OS X computer.
2. Search for Disk Utility in Launchpad and open it.
3. Select the Drive and click Erase.
4. Enter the new name (OPTIONAL).
5. Select MS-DOS(FAT) for Format.
6. Select Master Boot Record for Scheme.
7. Click Erase. (Depending on the USB Drive size, the operation might take some time.)