go to system management, get into the Xbox settings
see if the settings are set to show drive F:
it will either be a yes or no option, or a check box i forget which.
And are you sure you have a F: partition set up?
Sometimes people have F: and some other systems are E:
it all depends on how you modded the system.

Are you using the stock Xbox hard drive?
If so you will have very little space to store music, let alone games and movies.
It is only 8GB and most movie files are almost 1GB in size even when they are DivX. Games range from 1GB on up.
if you plan on storing media, and have a chip modded system. The best option is to upgrade the HDD to something 160GB or above.
Softmodded Xbox's you can do this too much it is a much more complicated process.
Did you check the settings i asked about?
If it says it should show F: then FlashFXP should show it too.
I know mine shows but it was also set up as the F: partition being my second one to store all my games, movies and music when i used an auto installer to set it all up after replacing the HDD with a 250GB one.