Hi,
I had problems a while ago with several USB sticks on Debian Jessie. For all non-root users they were automounted read-only. But there was no way of writing something onto the sticks for a non-privileged user. Fiddling around with user permissions and such didn't help. The simple solution finally was to comment out the following line in /etc/fstab:
/dev/sdb1 /media/usb0 auto rw,user,noauto 0 0
After commenting out this line all normal users could write to all USB sticks, just as you would expect it to work. Someone suggested that the fstab entry existed in the first place because I had installed Jessie from a USB stick as root, but I can't judge that.
If you look in the forums, many people face the same problem - they can read, but not write to a USB stick plugged in. The same solution as above works for them. Examples: