How to write protect a USB drive

Open Command Prompt (cmd) as Admin.
Run Diskpart:


List disks:

DISKPART> list disk

  Disk ###  Status         Size     Free     Dyn  Gpt
  --------  -------------  -------  -------  ---  ---
  Disk 0    Online          298 GB  7168 KB
  Disk 1    Online          931 GB      0 B
  Disk 2    Online         1862 GB      0 B
  Disk 3    Online         7702 MB      0 B

Select the required disk:

DISKPART> select disk 3

Disk 3 is now the selected disk.

Make it read only:

DISKPART> attributes disk set readonly
Disk attributes set successfully.


Leaving DiskPart...

To clear the read-only attribute:

attributes disk clear readonly

Note that the readonly bit is not absolute. Viruses can ignore the bit.