MacOS – Issue with SD card format – won’t unmount

The following procedure should format your SD card to make it usable again and cause bad sectors on the card to be remapped if that is part of your problem. Warning, erasing the wrong drive could make you cry so make sure that you know what you are doing.

1. Before inserting the SD card into your Mac, make sure that the write protect (lock) switch is turned off
2. Open a Terminal window and type diskutil list
3. Insert card
4. If an OS X messages pops up asking you to "Initialize... Ingore or Eject", choose "Ignore"
5. From the Terminal windows, type diskutil list once again
6. Examine the difference between the two Terminal outputs to determine the disk number for your SD card. You should be looking in the left most column for something like /dev/disk3 or 4 or 5 etc.
7. Now type diskutil unmountDisk /dev/disk# where # is the number you determined earlier
8. Here is where you must be VERY CAREFUL. Enter the following sudo dd if=/dev/zero of=/dev/rdisk# bs=64k and be sure to replace the # with the previous disk number. Note the 'r' in front of the rdisk in case you are typing this by hand instead of copy/pasting the text. You should be prompted for your password to run this command because it is dangerous
9. Be patient until it finishes. You can estimate time by opening the "Activity Monitor" application in Utilities and clicking on the "Disk" tab. At the bottom you should see the Data written/sec in red. It should be some number in MB like 5. (64 x 1000 / n) / 60 = minutes until it finishes where n is the rate in MB that you just determined or 5 in my example
10. Once the previous command completes, the card should be completely erased and ready for formatting. At this point you can close your Terminal and open the Disk Utility application to create an exFAT partition like you were trying to do earlier.

If you don't need to use this USB disk on a Windows machine, format using Mac OS Extended (Journaled). Should you need to exchange files with Windows machines, then Format as you have with MS-DOS (FAT). ExFAT should work as well.

It is not outside the bounds of possibility that the drive itself has gotten zapped somehow and is damaged, thus not operating correctly. Copy all the files from the flash drive to a folder, reformat with MS-DOS (FAT), then see what happens when you copy back. If it doesn't work, chuck it. Since flash drives are so inexpensive now, get a few more of them in larger capacities.

Lastly, (above) some flashdrives have a physical write-lock (read-only) hardware switch. If your flash drive has such a switch, perhaps it is locked? OS X does not allow the usual file & directory permission on MS-DOS (FAT) disks, so you can't try to make it read/write.

There is also a 4GB limit on a single file in MS-DOS (FAT).