Lost the 'permission' to write to a USB drive

My Mac mini (Mid 2011 model running OS X 10.10.3) HD is backed up by a Passport external drive and select files are stored on another (USB) drive. As the sole user of this Mac, I have been able to read & write to all three drives. Today, I tried to save a file to the USB drive and was advised "You do not have permission to save files to 'name of my usb drive' ". When I went to the USB drive / Get Info, the Sharing & Permissions reads: "You can only read" and, unlike the HD or the Passport, there is no lock icon in the lower right corner of this Get Info screen for me to unlock and make a change. What must I do to restore my ability to write to my own USB drive?