Q:

USB is locked (can't be written to or formatted) after using it to make a live Ubuntu installer

I wanted to try Ubuntu 18.04 so I used "make start-up disk"  to create a USB with the .iso on it (I'm using 16.04 - x64). I then used it to install 18.04 on a spare partition and all good.
Today I wanted to re-use the USB drive but it I cannot access it. 

I cannot write it.
I cannot add files to it as root.
I cannot reformat it with GParted.
I cannot reformat it with Windows Disk Manager
I cannot reformat it with Partition Wizard in Windows.

How can I recover this drive?

A:

You used dd or dd under the hood which only works with ISOhybrid created install ISOs. Ubuntu ISOs are isohybrid, Windows ISO are not.
https://help.ubuntu.com/community/mkusb/isohybrid
The dd or similar tools like mkusb then image the ISO to flash drive in a hybrid DVD/flash drive bootable mode. It is not a standard partitioned drive then but like a DVD. You generally then have to use dd to erase first sector or two, so partitioning tools will work. See
https://help.ubuntu.com/community/mkusb/wipe
For testing and regular reinstalling better to just directly use ISO. I download ISO directly to HDD first time only. Then update with zsync and directly boot ISO using grub's loopmount. I then install on SSD from HDD with toram parameter and it can be re-installed in about 10 min directly from RAM into existing (previous) partition.
https://help.ubuntu.com/community/Grub2/ISOBoot

