First off, is it possible to overcrank while using this hard-drive?
Yes. It will overcrank to the same speeds as the MxR, ie all HD formats, and under 720p up to about 60fps.

Does it require a hard-drive to be installed into it or does it already come with a hard-drive, and if so what sizes are available?

You will need to install a hard drive into the enclosure. On our website you can see the ones we have tested, but pretty much anything will do.

What kind of connections does the cable make between the hard drive and the camera? In other words, does it lock in somehow? Would this be a suitable solution for hand held, run and gun documentary style shooting or would it risk shaking loose?

We use a Mini-USB connector on the Expresscard and on the enclosure. On the enclosure side we have designed a custom leather bag which has a Velcro secured flap, which prevents the Mini-USb from being dislodged. Having said that, if the cable were to become dislodged, the camera will switch to the other slot to continue recording, so nothing would be lost.

But “run and gun” for any solution is pretty tough stuff. If I were going to do it, I would go for and SSD rather than an HDD, they can handle so many more knocks and the rough and tumble of what we do.