[ubuntu-uk] Strange file properties change

[Gordon Burgess-Parker]

10.04 LTS
I have just copied 3GB of folders, sub-folders and files to a flash drive.
The files in ONE sub-folder seem to have had the permissions changed 
from Read/write to Read only during this copy process. NO OTHER file or 
folder has been affected. The source folder and files are Read/write.
Can anyone explain why this might have happened?