Cannot edit tables or feature classes in a personal geodatabase

Description

When editing a personal geodatabase in ArcMap, some of the feature classes and tables cannot be edited.

Cause

The table has a unique index on a user-managed column. Tables with unique indexes on user-managed columns cannot be edited.

Solution or Workaround

1. In ArcCatalog, right-click the feature class or table and open the Feature Class Properties dialog box.
   
   
2. Click the Indexes tab and use the Delete button to drop the unique index.
   
   
   
   
3. Use the Add button to create a non-unique index.