"How can I unlock a free USB so I can write on it?"
Can I assume "a free USB" means a 2.5" drive? USB is the hole in the side of the Mac. Is the drive a thumber? If so, it may have a slide switch on the side to lock it. If it is not a thumb drive (cordless), read on.
It isn't the entire drive that is read-only (typo for you). It is the partition that is read-only. Solution? Remove the partition.
"I have a Mac."
Great, but what does your Mac have? OS X 10.6? 10.12? My answer is for 10.7.5.
If you need the files that are now on it, do this....
– Copy the files to another drive or to the Mac's internal storage.
– Open Disk Utility.
– Select the whole drive for the thumber (not the partition of it).
– Select the "Partition" tab (If there is no Partition tab, it means you didn't select the whole drive).
– Under "Volume Scheme > Current", select the partition's blue rectangle, and then click the - sign.
– Click the "Current" drop-list and choose "1 partition".
– Click "Apply".
– When that is done, select the "Erase" tab.
– Set the format to whatever you like, and click "Erase".