Here is a link that shows you how to reformat an SD card (or MicroSD card) on your Mac. The steps are also written out below.
Connect the SD card to your computer by using an external card reader. Save all the files from your SD card that you want to keep to your computer before moving to step 2. Formatting the SD card deletes all the data off of the card.
Note – If you’re using a Micro SD card, please insert the Micro SD card into the SD card adapter that came with card. Connect the SD card adapter to your computer by using an external card reader.
Also note – Mac OS 10.6.5 or earlier does not support exFAT format, which most 64 GB SD cards are formatted to. This means that you won't be able to format or use exFAT card on your computer. In this case, we'd recommend either upgrading your OS to 10.6.6 or later, or using an SD card that's already formatted to MS-DOS (FAT) or FAT32 to complete the below steps.
Open Disk Utility. To do this, click on the magnifying glass in the top-right corner of your computer screen. Type “disk utility” in the search box that opens, then click on the “Disk Utility” Application that comes up.
Find the SD card in the left side of the window. It’ll be named something like NO NAME or UNTITLED.
Click the ERASE tab in the middle of the window.
Next, click on the Format dropdown menu. Select MS-DOS (FAT) if the card is 32 GB or smaller. If it’s a 64GB card, please select ExFAT.
A message will appear asking, “Are you sure you want to erase the partition “[YOUR SD CARD NAME]?” Click Erase. The computer will delete the contents of your SD card and format it. You’re all done.
Step 1: Bring up Desktop View
If you are not already in Desktop View, bring up Desktop View by pressing the Windows key + D on your keyboard.
Step 2: Open 'Computer'
Open up the 'Computer' window by pressing Windows key + E on your keyboard.
Step 3: Display right-click options
Your SD card should be displayed in the 'Devices with Removable Storage' section. Right-click on your SD card to bring up the right-click menu options. Select Format.
Step 4: Format the SD card
In the Format window make sure that the correct File System is selected. It should be set to FAT32 for 32GB SD cards and EXFAT for 64GB SD cards.
Select Quick Format.
Once the formatting is complete, you can close the window.
Congratulations! You have formatted your SD card.
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