[solved] max Limit on Disk I/O with wmi?

[himo1]

Hi,
we monitor a Win 2008 R2 Server with the wmi from this topic:
http://forums.cacti.net/viewtopic.php?t=30438
Everything works fine, but on the Disk I/O for E: we have a "nan" at the value "writebytespersec" in the graph.

Is there a Limit on this value, because it´s high?
The monitor host is an Ubuntu 12.0.4 LTS with Cacti 0.8.8b

EDIT: After deleting an recreating the rrd file eveything works fine.