Perhaps they have special vessels to preserve stuff, as opposed to an all-encompassing defense as long as you're on the ring. IIRC, the rings were also responsible for the reseeding of the galaxy.

: Okay, so the Flood got loose a long time ago.. and the Covenant
: (accidentally) fed it enough to create a Gravemind... yet aren't we led to
: believe that the prophets are surprised and happy to find Installation
: 05("look at how our faith has been rewarded!"), like it's a new
: and recent thing? Did a Gravemind form in just a day or two after the
: Covenant landed?

Well, about the Prophet, you gotta remember that they were sniveling, conning bastards in H2. They wouldn't mention something as bad as "the disease is eating us up for breakfast" when trying to woo someone. But yeah, the timeframe is a little odd, considering Gravemind managed to capture 2401 some time before H2.