I'm trying to format an external disk (identified as /dev/disk4) using diskutil partitionDisk on macOS. However, I'm encountering the following error:

Error creating partition map: A writable disk is required (-69772)

Here's the relevant output from diskutil list:

/dev/disk4 (external, physical):
  #:                                 TYPE NAME                     SIZE       IDENTIFIER
  0:                            FDisk_partition_scheme        *1.0 TB     disk4
  1:                                 DOS_FAT_16                 76.5 MB    disk4s1
                                   (free space)                  1.0 TB     -

The disk seems to have a small existing DOS_FAT_16 partition (disk4s1) and a large free space area.

What could be causing this error and preventing me from formatting the disk?

I've tried:

diskutil eraseDisk free itsfree /dev/disk4


sudo diskutil partitionDisk /dev/disk4 1 GPT APFS "New Partition" 0b

More info about the disk:

diskutil info /dev/disk4
   Device Identifier:         disk4
   Device Node:               /dev/disk4
   Whole:                     Yes
   Part of Whole:             disk4
   Device / Media Name:       PSSD T7

   Volume Name:               Not applicable (no file system)
   Mounted:                   Not applicable (no file system)
   File System:               None

   Content (IOContent):       FDisk_partition_scheme
   OS Can Be Installed:       No
   Media Type:                Generic
   Protocol:                  USB
   SMART Status:              Not Supported

   Disk Size:                 1.0 TB (1000204886016 Bytes) (exactly 1953525168 512-Byte-Units)
   Device Block Size:         512 Bytes

   Media OS Use Only:         No
   Media Read-Only:           Yes
   Volume Read-Only:          Not applicable (no file system)

   Device Location:           External
   Removable Media:           Fixed

   Solid State:               Yes
   Virtual:                   No
  • What is the exact command that failed? Can you post the exact output? Commented Jun 11 at 18:45
  • What kind of disk is it, how's it connected, etc? What's the output from diskutil info /vol/disk4? Commented Jun 12 at 7:59
  • @GordonDavisson, here you have! thx
    – scd
    Commented Jun 12 at 9:46
  • The "Media Read-Only: Yes" line tells us the immediate problem, but as for why it's showing up as read-only... It might have something to do with the drive itself. From the Device/Media Name, it's a Samsung Portable SSD T7, which is an encrypted drive, meaning there's a fair bit of complexity to its firmware and operations. Do you need to install its software and configure it to set it to read-write mode? Commented Jun 12 at 10:20


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