If you mix first-generation (1st Gen) and second-generation (2nd Gen) parts in a DCU, the DCU might not boot, or might boot with warnings.
To troubleshoot these problems:
Know how mixed CMUs can conflict with mixed EMSs and HDD cages. See DCU Bootability With Mixed EMSs and HDD Card Cages.
Know how to identify 1st Gen and 2nd Gen parts. See Identifying 1st Gen and 2nd Gen Components.
Know which EMSs and HDD cages are incompatible. See EMS and HDD Cage Incompatibility.