MS doesn't place a F: drive on the partition.
EvolutionX is checking if you have an F drive and if you haven't it 0 MB.
When you put an 80 or 120 GB hd in your system you can format it with evoX and you will get an F: drive.
Since you have the original you have 0MB free.
About your question of getting 2 extra GB. I don't know and i never took the time to investigate. Once your stacking your xbox with divx movies and games 2 extra GB aint sh*t