If one solve in out-of-process mode (e.g., “My Computer (Background)”), then MODDIR is not used. This is because with RSM Workbench Mechanical can’t assume that you’re solving on the same machine, which means that the modal files may not be available to the solver running via RSM. Thus, when we use RSM, MODDIR is not used (modal files are copied) – solving “My Computer (Background)” is using RSM, so it also bypasses use of MODDIR.
We document this behavior below:
help/wb_sim/ds_response_spectrum_analysis_type.html
where the following is noted:
File Management When solving a Response Spectrum analysis in “In Process” solve mode, the pre-requisite files from the upstream Modal system are referenced by specifying the full path of their location (refer to RESUME and MODDIR commands) instead of making copies in order to improve solution time and disk usage. Please see the Solve Modes and Recommended Usage section of the Help for more information about the different solve modes. When you are solving in the “Out of Process” mode, the application copies the pre-requisite files from the Modal analysis to the Response Spectrum Solver Files Directory. This may increase the required solution time for large models.