In earlier days, the BIOS (little chip sized traffic cop that tells all the other chips and stuff how to interact with eachother) couldn't deal with a HD larger than 137 Meg. It's also referred to as LBA48. It made it necessary to have anything over 137 Meg to be stuck on "G" drive in order to be recoginized in the first incarnation of the hack.
That's been fixed (extra-hacked).
So, if you have a drive OVER 137 Meg, you need decide if you want the legacy way (a G drive with all the extra space over 137 Meg) or the "better" way with F reflecting the whole size of the HD you've installed.
I've got the legacy way working on my 160 Gig because I didn't want to have to re-format it and re-copy everything. IE: Legacy is good for those of us who upgrade our chips.
Does this help explain it?