I was wrong:
mount.nfs: access denied by server while mounting 192.168.0.3:/Volumes/500GB
ehowtons-mac-mini /sbin/nfsd[65]: Can't export /Volumes/500GB: Operation not supported (45)
Nothing I have tried has worked, and I've been at it nearly seven hours. There are a ton of resources online for how to mount ubuntu to OSX, but only a single one for the other way around. Not that it would matter, of course. NFS is NFS, and since the 'Operation' is 'not supported' I'm looking at going deeper. Some kernel-module switch flipped off perhaps. Here's the sole OSX NFS posting online which keeps showing up. I want to physically harm the person who posted it:
Setting up NFS exports under Leopard is insanely easy: just add an entry to (or more usually, create) /etc/exports, and it gets picked up automatically. This file survives reboots, as well; pretty cool.
Here's an example:
muse:~ root# cat /etc/exports
/Volumes/BigDisk/Panic -maproot=netroot 10.0.1.1
muse:~ root# showmount -e
Exports list on localhost:
/Volumes/BigDisk/Panic 10.0.1.1
muse:~ root#
"Insanely easy."