Thanks for the reply. It seems to have worked! Scratch that, it worked as long as I didn't connect an HDD. Once I do, the OS never comes alive (blue LED keeps on solid; no heartbeat). Removing the HDD doesn't help. It's as if the installation gets corrupted.
I then chose Buster Server 4.14 and that worked. Either the "minimal" build is creating a problem or Kernel 5.4.