I am always struggling to figure out how to fix “read-only permissions” when plugging an external USB into your Mac. If you are on my team, this has worked for me:

  1. Get the list of mounted volumes:
diskutil list

You will get a list of volumes, both internal (your hard disk(s)) and external (your USB).

  1. Identify the external one with read-only permissions (e.g. /dev/disk4) and unmount it:
sudo diskutil unmountDisk /dev/disk4

In my case it’s always /dev/disk4 :man_shrugging:

  1. Create a folder where the USB will be mounted:
sudo mkdir /Volumes/EXTERNAL_USB
  1. Mount the USB again with write permissions:
sudo mount -w -t exfat /dev/disk4s1 /Volumes/EXTERNAL_USB


  • -w stands for write permissions.
  • -t exfat is the external type. It should match the format you used when formatting your USB (e.g. exfat, nfs, msdos, etc).
  • /dev/disk4s1 is the disk identifier + partition to mount.
  • /Volumes/EXTERNAL_USB is the folder you created in step 3.

The tricky thing to remember is the external type you should use. Valid options can be found by running:

ls /sbin/mount_* | cut -c13-

And that’s all. Hope it saves time for you, and it will save time for me from now on :laughing:.

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