Why does Google Chrome mount and unmount a volume every few hours?
Apparently there is a Google Software Update process that is queued to run periodically, which is why it's not always there. (I couldn't grab a screenshot of it, it was too sneaky). It's not just for Chrome, but for Google Earth and Picasa too. It was a bit of a breadcrumb hunt finding info about it.

I think the Google updater isn't a proces that runs all the time, but a launchd job that runs when you boot and every so often after that. Google com.google.keystone.user.agent and you'll find a number of explanations of what they've done (and how to get rid of it if you want).
charlie  (Apple Discussions)

From Google's Help Page:

Google Software Update is a background application for the Mac OS that helps ensure that you always have the most up-to-date, stable, and secure versions of the Google software you have installed. Google Software Update may run two services: GoogleSoftwareUpdateAgent and GoogleSoftwareUpdateDaemon. These services allow Google applications to be safely and securely updated.

I couldn't find, and I'm not aware of any reason it mounts a partition, but it's probably either:

A bunch of information about which updates are available and for what apps
Updates for products, Google Chrome updates are quite regular
An Update for the Updater

Ah the endless mysteries of HardwareGrowler. Entertainment for the whole family!