Disk Write Error

When trying to tansfer files from PC to USB Drive i get the error "The disk cannot be written to becasue it is write protected. Please remove the write protection from the volume in drive \Device\Harddisk1\DR3"

My usb drive does not have a lock located on the drive and this does not happen until I am almost thru backing up a folder that is over 500 MB large, it writes up to a certain point then errors out. It is a 1gb drive with 360 mb free.