diff --git a/app/assets/stylesheets/application.css b/app/assets/stylesheets/application.css
index 4c49e38..14ec9a7 100644
--- a/app/assets/stylesheets/application.css
+++ b/app/assets/stylesheets/application.css
@@ -410,7 +410,8 @@ table.items td.link a:hover:focus-visible {
   color: #006c9b;
 }
 
-table.items td:not(:first-child) {
+table.items td:not(:first-child),
+.grayed {
   color: var(--color-table-gray);
   fill: var(--color-table-gray);
 }
diff --git a/app/models/unit.rb b/app/models/unit.rb
index e9025f8..53af7df 100644
--- a/app/models/unit.rb
+++ b/app/models/unit.rb
@@ -19,7 +19,7 @@ class Unit < ApplicationRecord
     bases_units = arel_table.alias('bases_units')
     other_units = arel_table.alias('other_units')
     other_bases_units = arel_table.alias('other_bases_units')
-    parent_units = arel_table.alias('parent_units')
+    sub_units = arel_table.alias('sub_units')
 
     Unit.with(units: self.with_defaults).left_joins(:base)
       # Exclude Units that are/have default counterpart
@@ -32,20 +32,30 @@ class Unit < ApplicationRecord
               .and(other_units[:symbol].eq(arel_table[:symbol]))
               .and(other_units[:user_id].not_eq(arel_table[:user_id]))
           ).exists
-      # Decide if Unit can be im-/exported based on existing hierarchy
-      ).joins(
-        arel_table.create_join(parent_units,
-                               arel_table.create_on(
-                                 parent_units[:symbol].eq(bases_units[:symbol])
-                                   .and(parent_units[:user_id].not_eq(bases_units[:user_id]))
-                               ),
-                               Arel::Nodes::OuterJoin)
+      # Decide if Unit can be im-/exported based on existing hierarchy:
+      # * same base unit symbol has to exist
+      # * unit with subunits can only be ported to root
       ).select(
         arel_table[Arel.star],
-        Arel::Nodes::IsNotDistinctFrom.new(parent_units[:symbol], bases_units[:symbol])
-          .as('portable')
+        arel_table[:base_id].eq(nil).or(
+          (
+            Arel::SelectManager.new.project(1).from(other_units)
+              .join(sub_units).on(other_units[:id].eq(sub_units[:base_id]))
+              .where(
+                other_units[:symbol].eq(arel_table[:symbol])
+                  .and(other_units[:user_id].not_eq(arel_table[:user_id]))
+              )
+              .exists.not
+          ).and(
+            Arel::SelectManager.new.project(1).from(other_bases_units)
+              .where(
+                other_bases_units[:symbol].eq(bases_units[:symbol])
+                  .and(other_bases_units[:user_id].not_eq(bases_units[:user_id]))
+              )
+              .exists
+          )
+        ).as('portable')
       )
-      # complete portability check with children
   }
   scope :ordered, ->{
     left_outer_joins(:base)
diff --git a/app/views/default/units/_unit.html.erb b/app/views/default/units/_unit.html.erb
index 53d8889..19af7a3 100644
--- a/app/views/default/units/_unit.html.erb
+++ b/app/views/default/units/_unit.html.erb
@@ -1,16 +1,17 @@
 <%= tag.tr do %>
-  <td class="<%= class_names({subunit: unit.base}) %>">
+  <td class="<%= class_names({subunit: unit.base, grayed: unit.default?}) %>">
     <%= unit.symbol %>
   </td>
 
   <td class="actions">
     <% if current_user.at_least(:active) && unit.default? %>
       <%= image_button_to t('.import'), 'download-outline', import_default_unit_path(unit),
-        disabled: !unit.portable? %>
+        !unit.portable? ? {disabled: true, aria: {disabled: true}, tabindex: -1} : {}  %>
     <% end %>
     <% if current_user.at_least(:admin) %>
       <% if !unit.default? %>
-        <%= image_button_to t('.export'), 'upload-outline', export_default_unit_path(unit) %>
+        <%= image_button_to t('.export'), 'upload-outline', export_default_unit_path(unit),
+          !unit.portable? ? {disabled: true, aria: {disabled: true}, tabindex: -1} : {}  %>
       <% else %>
         <%= image_button_to t('.delete'), 'delete-outline', unit_path(unit),
           method: :delete %>