1) Some drives have a little slider switch for write protection. Examine your drive closely to see if there is one.
2) If not, a look at the recent forums will show you at least a hundred such threads. Flash drives seem to be simply unreliable. While some cases are fixed with registry policy changes or resolving other obscure issues, the majority require that you copy all the data off the device and then reformat it (see near the end of http://www.tomshardware.com/forum/272402-32-flash-drive
Am I missing something, or are these devices failing a lot, or are there simply so darned many of them out there that a reasonable failure rate translates to a crapload of failures?