Free disk space is only one reason. Check these.

1. Autogrowth. Open database properties > Files. If autogrowth is off or the file has a maximum size, SQL Server can not grow even when the disk is free. Turn it on and use a fixed growth size, for example 512 MB, not a small percentage.
2. Disk. Make sure the drive that holds the data file has space. A different drive than you looked at may be full.
3. Quota or mount point limits on the volume.
4. Look at sys.database_files and compare size, growth and max_size.

If the file is at its maximum, raise MAXSIZE with ALTER DATABASE db MODIFY FILE (NAME = 'logical_name', MAXSIZE = UNLIMITED); and try the insert again.