diff --git a/openbis/source/java/ch/systemsx/cisd/openbis/generic/server/business/bo/DeletedDataSetTable.java b/openbis/source/java/ch/systemsx/cisd/openbis/generic/server/business/bo/DeletedDataSetTable.java
index 4f1a7de02b3702d8b803ad9bdf2f17ee709e61bb..9c1ded565823b50e8d20a9829d70afc25ea5d653 100644
--- a/openbis/source/java/ch/systemsx/cisd/openbis/generic/server/business/bo/DeletedDataSetTable.java
+++ b/openbis/source/java/ch/systemsx/cisd/openbis/generic/server/business/bo/DeletedDataSetTable.java
@@ -183,6 +183,7 @@ public final class DeletedDataSetTable extends AbstractDataSetBusinessObject imp
             throw new UserFailureException(
                     "The following data sets are unknown by Data Store Servers they were registered in. "
                             + "May be the responsible Data Store Servers are not running.\n"
+                            + "If you are sure that you want to delete the data, try to Force Empty Trash by clicking on the button when holding Alt key.\n"
                             + unknownDataSets);
         }
     }