you will need to edit your xboxmediapalyer.xbl
open it with notepad
first look where your f drive is pointing eg F:\ movies
should look some thing like this - Note this is from my xbmc xml so might not be exactly the same.
<!-- my video shares !-->
<video>
<default></default>
<bookmark>
<name>E Drive</name>
<path>E:\</path>
</bookmark>
<bookmark>
<name>F drive</name>
<path>F:\</path>
</bookmark>
<bookmark>
<name>G drive</name>
<path>G:\Video</path>
</bookmark>
i am sharing my G:\video
hope that helps

you might need to map the drive as well
should look some thing like this -
Tried <map>G:,Harddisk0\Partition7</map> dont think this 1 is right
just copy your f drive 1 and paste it under , change f to g