Well my bios does support 48bit addressing for HardDrives to overcome the 137gig limit. I do not have my xbox with me today or i would try it.
I would like F: to be bigger. The bios does have 2 choices = F: gets all or F: and G: drive combo. If I can set the size of G: would it be an issue is F: is over 137g ?
Regardless of what your bios gives you the option to do, if you have already formatted the HDD to be all F, you'll have to reformat both F and G to get your G drive working, even if you haven't filled your F drive to 137GB of data.