USB Read-Only Filesystem
I wanted to extend john smiths answer. I had made two USB sticks bootable using Ubuntus "Start media creator". It made the sticks unusable afterwards, because they were write protected and I could not format them in Ubuntu or Windows. I tried fdisk, gparted. No dice.
What helped was inserting the stick (make sure this is the only USB drive inserted), finding out where it is mounted:
df -Th
That will show you a list of devices/partitions and their mount paths/points:
udev devtmpfs 7,8G 0 7,8G 0% /dev
...
/dev/sdb1 vfat 7,5G 4,0K 7,5G 1% /media/<USER_NAME>/<STICK_NAME_OR_ID>
Find your USB stick and remember its partition device path (here: /dev/sdb1) and mount point (here: /media/USER_NAME/STICK_NAME_OR_ID). Then unmount it:
sudo umount /media/USER_NAME/STICK_NAME_OR_ID
Now recreate the file system (FAT32 in this case):
sudo mkfs.msdos -F 32 /dev/sdb1
Wait a short time and unplug the stick. Wait again and plug it in. It should be writable now...