Comment 1 for bug 1014813

The USB key has a weird partitioning scheme. Just for the sake of completeness, could you post the output of sudo fdisk -l /dev/sdc so we can see what the USB key's partition table contains?

Also, output of ls -la /media/ would be useful to see the actual permissions for the directories where the USB key's partitions are mounted.

Finally, the following bugs are related to this problem (multiple partitions and writableness on USB sticks), although they are not duplicates:

https://bugs.launchpad.net/checkbox/+bug/887049
https://bugs.launchpad.net/ubuntu/+source/checkbox/+bug/912522

The second one in particular has a lengthy discussion about the exact problem (some partitions on USB stick unwritable). I guess this bug is about the "NO partitions writable" corner case :)