When an individual zone in a Fluent cas-file has a very large number of cells, then the following errors might occur when opening the cas, dat or cdat in CFD-Post: Windows: severe error operation failed. (system exception: vector too long) Linux: SEVERE Operation failed. (System exception: vector::_M_default_append) This indicates that one of the zones is too large to be opened in CFD-Post. A workaround for this issue is to split the largest zone. This can be done in Fluent with the Domain > Zones > Separate > Cells … command. The steps are: 1) create register to be used for separating the cell zone, i.e. by coordinate-direction 2) separate cell zone 3) save cas & dat, export cdat for CFD-Post 4) open cdat in CFD-Post