All USB memory sticks are write protected (12.04)

Hello.
All of a sudden all (4) my USB memorysticks are writeprotected, at least it is what my ubuntu 12.04 tells me when trying to write to them. All of them have been working fine before on my ubuntu 12.04 and this computer. There is physical protectionswithes on two of them but they are off. I can read and delete on them but not write to them. The system (12.04) is fully updated. I have three USB-ports but it is the same problem on all of them. It all worked fine a couple of days ago and I have not done any changes myself.

Regards
Sven