Note: If you have a write-protected SD card, check that the write-protect (LOCK) switch on the side of the card has not been accidentally moved to the WP (Lock) position! Some USB flash drives also have write-protect switches (sometimes in the end of the drive and they are very small!) and so do the Zalman VE200/300/400 ISO emulator HDD caddies.
Some USB Flash Drive (UFD) utilities can write-protect the whole storage area by modifying the firmware. For instance the "UFD Utilities" application can Write Protect or Un-Write-Protect most UFDs that have an SMI controller (such as Lexar UFDs). You can also make a CD/DVD-ROM partition on the drive using this utility.
If you have used a USB disk run the Windows 8 to Go! Operating System or perhaps a Vista/Win7 OS and now you cannot access it under Windows Vista or 7 even with RMPrepUSB, or even format it under Windows, it may be because the disk has been marked as Read-Only. You may be able to use it in other PCs or notebooks (tip: try anotehr system) but not your PC. Another test would be to try it on another Operating System (e.g. linux or your TV or media centre, if you have one).
I had just such a USB hard disk which had been formatted as NTFS and used as a Windows 8 ToGo! boot disk. I could not copy any files to it or format it under Windows 7 or wipe it using RMPrepUSB - every time I tried I got a 'Write-Protect' error.
To clear this I use RMPrepUSB - RMPrepUSB v2.1.655 and later now will run this for you on the selected drive. Use the Drive - Clear ReadOnly Status option in the drop-down menu (see below). This is not supported in Windows XP.
Or, if you prefer the hard way, you can use Windows 7 Diskpart as follows:
If you prefer to do this manually, then...
1. Start Menu - All Programs - Accessories - Command Prompt - right-click - Run as Administrator...
2. When the command console opens, type DISKPART
3. List the drives by typing LIS DIS
4. Select the USB drive by typing SEL DIS 1 (if disk 1 is your USB drive)
5. Inspect the details for that disk by typing DET DIS
Check if the disk is marked as Read-only - e.g.
Note: It is also possible to mark a volume as readonly, so try LIS VOL and SEL VOL F: (where F: is your USB drive letter) and then DET VOL to list the attributes.
If it says 'Read-only: NO' in Diskpart but shows as 'Read-Only' in Disk Manager, then the disk is probably physically write-protected (or perhaps write-protected by the firmware in the device's internal controller). In this case the following procedure will have little affect! You should look for a write-protect (aka 'Lock' or maybe a padlock symbol) switch on the device. If you are sure that none exists, consider re-programming the controller firmware.
6. Type ATT DIS CLEAR READONLY to clear the ReadOnly attribute and then type DET DIS again to check it has been cleared.
This also changes the Windows Registry (on the system you are using) to set the device to a Read-Write status. The command will thus appear to work even if the USB disk is physically write-protected!
(optional - if the drive still is Readonly try this) type CLEAN to erase all contents from the drive (make sure you have selected the correct drive!!!!!)
7. Finally type EXIT to quit Diskpart
8. Now unplug the USB disk and re-insert it into the USB port.