What happens when the ndf has a max size, but the mdf has unlimited? Will it start populating the mdf again once the ndf is full? Common sense tells me no, but I just need to validate. Thanks!
That depends upon how you set up the filegroups, etc. If you have only the PRIMARY filegroup and just files under that, the data will be loaded into all of those files including the mdf. So yes, the mdf file would get filled.