I get this once in a while, when I forget to eject the drive properly. First, look at the USB drive and see if it has some kind of switch. This usually toggles read-only mode.
If that doesn't do it, the easiest fix (for me) is to just reformat the drive.
Back up anything on the drive that you want to save, before continuing.
Just copy things to your desktop, this only takes a minute and you can put the files right back.
Open the dash, it's at the top left of your screen like this:
Type Disk Utility, like this, and select it to open the program.
- Find the drive in the list on the left
- Select the partition to be reformatted, in this example there is only one partition.
- Unmount the Volume (you can't work on mounted volumes)
- Select Format Volume
This opens up a window where you choose the type of volume.
- Go to the pull-down menu that says Ext4.
- Change it to FAT
- Click Format
Then, click Format on the confirmation dialog to continue.
This should solve the problems now. Good luck.