Can I perform a mass update for leads of different business units?

You can't perform a mass update of modified values for leads that apply to different business units. A dialog box is displayed if the selected leads belong to different business units which don't share the same reference data sets.

You must remove the set-enabled attribute for the offending leads and perform the mass update only for leads with business units containing the same reference data sets. For more information on reference data sets, see the topic How Business Units Work with Reference Data Sets. Also, consult the online help, using keywords "reference data".