The key setting is "Export JBOD Disks" under controller policies in the 3ware setup screen. This is accessed by pressing 3 or alt-3 after boot. If this is set, any virgin disks will by exported as JBODs automatically.
What if someone's already configured your disks into units, either single-disk or otherwise? You'll find that deleting the units doesn't turn them into JBODs. They just sit there as "available" but the OS can't see them. As far as I could find, there's no documented way to get them back to the pristine state. I think this is pretty rotten of 3ware.
The solution is to use the 3ware setup screen and the letter J. Get the available drive selected and press J. Save changes and reboot. Not documented, but effective.