This is certainly not good. NCS stops recording when it is less than 250 MB left on the HD. However, NCS should of cause follow the setting of the “GB rule” and start to overwrite old files. This was a bug and should be addressed in version 1372 of NCS. Do you use that? Is the HD an internal HD in the computer with a drive letter?
To delete files after a week use the setting for Retention period. This setting is valid for all recordings and is not selective to certain cameras. For long term storage on a NAS I think you need to experiment to find what is best for you. In the configuration of each cam you can set up an FTP function. In the rule manager you can also set an FTP function. There are also many small programs that manage files and folders based on rules. I sometimes use File Juggler for this and Folder Spy is another one, but there are many more. However, deletion of recordings must be handled by NCS otherwise it will be a big mess since all files are stored in a database. So an external program can only be allowed to copy files from the NCS Library.