This happened to me today, all of a sudden usb sticks are read only, what? Anyway using 17.3 Cinnamon, tried various methods found on forum, to fix, but what worked was to take it to a windows 7 machine and delete partition, create new volume and choose slow format and fat32. This worked. Anyway I wonder if the hardware section in control centre, (and I'm surprised it is spelled "center" from an Irish based distro
, needs a usb format and permissions section, or a better usb stick formatter with options for slow format. There seemed no way I could change permissions, I tried opening file manager as root, choose usb stick permissions tried changing there, but no go. There may have been a way via terminal but didn’t find that information, the thing is, there is nothing wrong with the sticks, they worked in other Mint machines and Windows 7 machine, just one Mint machine it did not work on that has recently updated from 17.2 to 17.3 via update manager.