I'm not sure if this applies, but did you check your config files of XBMC and MXM? When you updated, they might have been overwritten. For XBMC, in the XboxMediaCenter.xml file theres a spot that says;<useFDrive>yes</useFDrive>
Make sure those are set to yes...
I'm not familiar with MXM, but from what I've read I think it's supposed to auto-detect your drives. - so it would seem that it might be something with the bios. You could always try going back to the old versions of XBMC and MXM you were using and see if you can see the G drive then, if so, update each one at a time and see if the G drive vanishes after one of the updates, then you'll know if it's something with one of the new versions of the software...
If your G drive is still missing after reverting back to the old versions, then you'll know it's not with the new versions and you can look elsewhere (ie .. bios) for the problem.
Hopefully not all of my input is elementary info for you, and helps a little.
Good luck with it