When inverting CSEM data (i.e. from dipole sources and not MT data), MARE2DEM uses scratch files to temporarily store the large arrays of wavenumber domain sensitivity kernels, which are later read in (as well as deleted from disk) and then Fourier transformed into spatial domain sensitivity kernels.
By default, MARE2DEM writes scratch files to the /tmp folder that is local to each MPI processor. You can override this in the "mare2dem.settings" file by adding this line and replacing /tmp with another folder:
scratch folder: /tmp
The /tmp folder should work on most Unix based systems. For optimal performance, the scratch folder should be on the local hard drive for each MPI processor and should NOT be a network shared folder, since in that case the large amount of data written to the scratch files could overload the network used by the MPI communicators.