USB flash drive "write protected" error

[racho]

Yesterday I copied some pics to my flash drive, it worked well. But today I get write protected error.
I can't delete/change anything but I can copy them. There is NO switch on it. I tried chkdsk, says write protected too.
Same problem on other comps too.
I searched for this problem, nothing helped.
This http://www.computerhope.com/forum/index.php?topic=42095.0
is similar but I have another manufacturer.
I tried booting from ubuntu live cd, again write protected errror. So no viruses/os can be the problem.

specs: 256mb
Adata, www.adata.com.tw nothing else is written on it so I don't know the model.
No good support on their site.
256mb isn't much but I need it.
Please help.

[racho]

SOLVED!
This may help you if you have such problem.
I booted again to Ubuntu (why?) and formatted it. Suddenly it worked.
Don't know how I or Ubuntu did it, but it works under windows too.
I am sure have no viruses.
Linux forever.
M$ *****