Just to update the list for the archives and future searchers: The probes
look to be from udisks-daemon, as they start in the boot process immediately
after it is invoked by the discovery of an SD card in my case. It runs under
udev, and is a successor to HAL for monitoring media and such. Once active,
it also repeatedly - at least in the version distributed with Ubuntu 10.04 -
polls all other media, including the sectors of hard drives where GPT would
be, regardless of whether GPT are actually used on those hard drives.
So when a sector reserved for the secondary GPT became unreadable in my
case, udisks-daemon's polling of the sector (which was the only use that
sector had ever had, so could have led to its failure in the first place)
continued so aggressively as to retard boot by several minutes, and steal
resources from the system after boot. I've filed a bug with Ubuntu. It's not
the only distro using udisks-daemon though.