Building a network dataset in ArcCatalog or ArcMap occasionally causes a file read/write error to occur.
This problem occurs sporadically.
A reference to a temporary file is not being properly accessed.
This issue has been resolved with the service pack: Network Analyst 9.1 – Network Dataset Patch. Click on the link below and follow instructions to download.
If downloading the patch is not available to you, repeat building the network dataset until it is successful.