To reverse engineer an existing database into a relational model, I used SQL Developer Data Modeler, a free data modeling and database design tool from Oracle.
I had a problem with the tool. I could not save the model. It appeared to be saved but when I reopened the .dmd file, the relational model was nowhere to be found.
I tried all kinds of combinations on my Windows 7 64-bit laptop, like using JDK 6 vs. JDK 7, 32 bit vs. 64 bit versions, etc. No luck.
Then I stumbled upon this Oracle Forum thread while searching for a solution online. The poster suggested that enabling support for version control in the tool solved the issue.
I had versioning support disabled in SQLdev Data Modeler.
Following the hints in the forum post, I enabled it (Tools > Preferences > Extensions > toggle Versioning Support). Restarted SQLdev Data Modeler, and voila! I can now save my relational models!
There was no way I could have guessed that versioning support was interfering with saving relational models. I am guessing this is a bug.