When plug in an USB stick it happens with Ubuntu that it gets mounted read-only. Even with rw mount option it is not possible to write. Often the mount point directoy is not writeable (or even accessible) to the user. This could be solved by setting the ACLs.
To see the actual access rights
$ ls -ld /media/<username>
drwxrwx---+ 2 <username> <username> 4096 Mar 4 18:32 /media/<username>
where the + charcter indicates the use of ACL. These ACL can be shown as followed
$ getfacl /media/<username> # file: <username>/ # owner: <username> # group: <username> user::rwx user:<username>:r-x group::--- mask::r-x other::---
In this example, the user is read only. This can be easily changed to read-write (rw)
setfacl -m u:<username>:rwx /media/<username>
Eventually you have to reset the dirty-bit of the FAT filesystem of your USB stick.