How do I use MacOS Terminal or other app to format an MBR FAT32 SD card as a 0x0C partition, rather that 0x0B? Apple's Disk Utility app can format an SD card as MBR FAT32; but in the Terminal, I confirmed the SD card's partition is in fact 0x0B:
&gt; sudo gpt show /dev/disk3

 start      size  index  contents
     0         1         MBR
     1      2047         
  2048  62746624      1  MBR part 11

&quot;MBR part 11&quot; refers to 0x0B.  I want the partition to be MBR part 12 (0x0C).
I then tried this in the Terminal:
&gt; diskutil unmountDisk /dev/disk3
Unmount of all volumes on disk3 was successful

&gt; sudo fdisk -ia dos /dev/disk3
fdisk: could not open MBR file /usr/standalone/i386/boot0: No such file or directory

    -----------------------------------------------------
    ------ ATTENTION - UPDATING MASTER BOOT RECORD ------
    -----------------------------------------------------

Do you wish to write new MBR and partition table? [n] y


But if I click the &quot;Initialize...&quot; button, it merely opens the Disk Utility app, which I know will only format as MBR FAT32 0x0B.
How can I use MacOS Terminal or another Mac app (not Windows or Linux) to format the SD card as MBR FAT32 0x0C?
Your fdisk invocation should have already partitioned it as type 12. You can press the Ignore button and verify via Terminal that that's the case, then simply eject the disk from your computer and use it wherever you intend to use it.
If you intend to use this drive on your Mac, you will need to format and mount it after running fdisk:
sudo newfs_msdos /dev/disk3s1
sudo mkdir /Volumes/FLASH
sudo mount -t msdos /dev/disk3s1 /Volumes/FLASH

Here FLASH is an arbitrary volume name that I chose for sample purposes; feel free to choose your own.
As a clarifying point, the s1 portion of /dev/disk3s1 targets the first partition of disk3. If you were to specify just disk3 instead, the newfs_msdos command would overwrite the partition table.