Hiya Michelle. Maybe there actually isn't enough room.
Open Windows Explorer and find your external drive, which should be prefixed by a letter such as E: or F: etc., then right-click on the drive and select Properties. This will show, not only the capacity of the drive, but also how much free space remains.
Also check that your external drive hasn't been partitioned into smaller 'compartments'. This will become apparent if Explorer shows other letters for drives you never knew you had.