So I created a Data Source in WebSphere Application Server for testing my portlets. It was using the embedded JDBC Derby database, and pointed it to a file location on disk. Next I created the database with the ij.sh tool under ${WAS_ROOT}/AppServer/derby/bin. I remained connected to the db while I tested the connection and got this error:

The test connection operation failed for data source PortletDB on server nodeagent at node stgdom02 with the following exception: java.sql.SQLException: Failed to start database '/opt/IBM/WebSphere/wp_profile/databases/portletdb', see the next exception for details.DSRA0010E: SQL State = XJ040, Error Code = 40,000. View JVM logs for further details.

The problem was that I was already connected with the ij.sh tool. You can only share the connection within the same JVM. If you are not connected, the following may apply to you too, because I *did* restart WAS and Deployment Manager before my *doh* moment:  Test Connection fails with error DSRA0010E (and it does apply to WAS 7.0 as well).