This error comes if the disk volume where indexes are stored is getting FULL. It has nothing to do with tape media capacity. Check on the available free space on the volume where you are writing indexes. By default, indexes are written in the /index folder of the NW installation path untill you havce changed it. So, if NW is installed in C:\, C:\is running short of space.

To check exact index location, you can also refer to the Index Path attribute of the clients. If blank, it is using the default fodler else it would have the path there.

Create more space on the Index path so that freshh indexes can be written.