I would use diskpart if you are running Windows 7 (the syntax varies slightly for other Windows versions):

Open the command prompt box with right click and run as administrator

Type each of these commands one at a time in this order:
list volume
select volume n (where n = the external volume)
attributes disk clear readonly
attributes volume clear readonly

After each of the last two you should get a response that it was successful.

Your drive should now be both read and write capable.

Here is a reference: