You should be using a commercial Microsoft SQL Server instance with your NTA v3 (or earlier) installation.
Express Edition is definitely problematic for handling NetFlow data, not just because of the space limitations, but also because of the performance limitations.
With NTA 4.0, however, we provide a new datastore that does not require a SQL Server license (but does require an x64 server).
I would encourage you to upgrade to NTA 4.0 and implement the new datastore technology.
Correct :-) In case you will decide to go with NTA 4.0 while using the SQL express DB please check following KB: SolarWinds Knowledge Base :: Installing NTA 4.0 Evaluation with Remote NTA Flow Storage: How to Get Orion Database Crede… it will guide you how to obtain needed SQL credentials.