probably because the folder below it are not writeable.
if using fat32 – you should make sure it is writeable to the user you will be using. this can be done with mount options.
if mounting via fstab
http://serverfault.com/questions/43634/how-to-mount-external-vfat-drive-as-user
if using usbmount
http://unix.stackexchange.com/a/39941
