For some time I have used a single core, and "dataimport" worked well. But when it took two different index, I installed the second core, and there was an error:
org.apache.solr.common.SolrException: Error loading class 'org.apache.solr.handler.dataimport.DataImportHandler'
You need to make sure the core can access the dataimport jar files. This easiest way to ensure this is to put the jar file in coredir/lib , where coredir is the dir pointed to as instanceDir from solr.xml.