Error: no elements in volume 1
I'm trying to create a mesh for a rectangular box with a cylinder completely embedded inside (no shared boundaries). The two regions (cylinder and everything else) have different material attributes, but there are no special boundary conditions for a finite element code. I simply want the mesh to be concentrated along the border.
Attached is my attempt. The mesh seems OK when I view it, but the gmsh log window says
Error: no elements in volume 1
It looks like volume 1 was somehow defined but I don't know enough about gmsh to know what volume 1 actually is.
I also can't tell from the documentation when a numerical tag (1, 2, ...) can be re-used for lines vs surfaces vs physical surfaces vs surface loops.... For example, when I give the volumes with the tag 8 in the file the tag 1, I get an error that that tag has already been used.