Making virtualbox drives bootable. [SOLVED]

[cwr]

Does anyone know how (or if) you can make VirtualBox drives (ie. file images)
bootable? Ubuntu, Fedora, and so on can be installed from LiveCDs which
seem to set the VirtualBox virtual drive up automatically. Installing Gentoo
from a Gentoo minimal CD and a stage 3 file works fine as far as installing
goes, but apparently doesn't set up the virtual drive in the way VirtualBox
expects it. (It's not just a matter of the boot flag not being set.)

At the moment all I can get is the
"FATAL: No bootable medium found! System halted"
message, andI can't find a way of telling VirtualBox that the image is bootable.
Any ideas?

Thanks - Will

Later: the solution is to install from a Ubuntu CD, not the Gentoo minimal CD,
so that you can use gparted to initialise the virtualbox image and not cfdisk.
Both tools mark the partition as bootable, but apparently only gparted sets
up the boot process/mbr correctly.

[Rexilion]