How To Remove Write Protection From Pen Drive?

1. Go to run, type regedit and press enter.
2. Navigate to the path:
HKEY_LOCAL_MACHINESYSTEMCurrentControlSetControlStorageDevicePolicies
If the registry key StorageDevicePolicies does not exist, you will need to create it manually.
3. Highlight StorageDevicePolicies, and then create a New DWORD (32-bit) Value named as WriteProtect.
4. Double click the key WriteProtect in the right window and set the value to 0 in the Value Data Box and press OK button
5. Restart your computer and see if this help

If there is no important file, try to format
Click on Start.
Type diskmgmt.msc in Run.
In the disk management window, select the pen drive, right click on it and click on Format. Now choose the File System and Allocation Unit Size.
Click on OK.
or
1. Start > search bar -> CMD and press enter
2. type< diskpart
3. type< list disk
4. type< select disk # (# is the number of the USB key)
5. type< attributes disk clear readonly

perhaps this hot-fix can help also
http://www.makeuseof.com/answers/remove-write-protection-sandisk-pen-drive/
perhaps take ownership
http://www.rizonesoft.com/2011/ownership-shell/
You can find two reg files below if you are on xp or vista
http://www.mydigitallife.info/how-to-disable-write-access-to-usb-hard-disk-and-flash-key-drives/

How To Install Window 7 By USB.

Install Windows 7 From USB Drive [Requires 2 Simple Steps]
I have found a much easier way to install Windows 7 from a USB Flash drive. Unlike other methods where you have to write complicated commands, this method can be completed even by those who have very little computer background.

The whole process takes only two steps, run UNetbootin, load the Windows 7 ISO file, and finally restart your computer. See how we installed Ubuntu from USB using UNetbootin here.
Before you begin, you will require the following:

• USB Flash Drive (4GB minimum)
• Windows 7 ISO Image file
• UNetbootin (download here)

Note: If UNetbootin doesn’t work, try out the Microsoft’s official tool called Windows 7 USB/DVD Tool.

Now insert the USB drive, run UNetbootin, and select Disk Image as ISO. Browse your local drive for Windows 7 ISO that you downloaded and click Open. Now Select Type as USB and choose the drive. Once done, it will look like a bit similar to the screenshot shown below.

Click OK and it will begin extracting all installation files to the USB drive. The whole process will take some time(10-15 minutes), so have patience.
Once the installation is complete, reboot your computer. Now while your system is starting up press the appropriate button(usually F1, F2, F12, ESC, Backspace, or Escape) to bring up Bios Boot Menu. Change the startup order to boot USB by default, usually you will have to press F6 to move the selected USB device on top. Once done, save changes and restart the system.