Since it's FAT32, the "permissions" mean that the disk is mounted as read-only (since FAT disks don't have any permissions).
Hmm... there are a few reasons why this happens. What kind of flash drive is it?
One thing to check is to make sure there isn't a pre-existing folder at the mount point. Eject / remove the drive and then open /Volumes in terminal and make sure that there is no directory there for the USB drive while the drive is not plugged in. If the directory is there without the drive, delete it (probably will need to sudo rm it).