Problem was fixed by going to Control Panel/Services/SQL Server (SQLEXPRESS) Properties and changing Logon to Local System Account.

Thanks for your insight, but you are getting a bit ahead of my knowledge base.

I installed SQLEXPRESS using Windows Authentication. This is a simple, 3-computer network that doesn’t use...

Error response: Directory lookup for the file "E:\SQL_DATA\test.mdf" failed with the operating system error 5(Access is denied.)

Yes, external drive is a USB.